IFD (indentation force deflection), formerly called ILD (indentation load deflection), is the force needed for a flat circular indenter foot of 323 cm² (50 in²) to compress flexible foam by 25% of its thickness, read 60 s after the deflection is reached. ASTM D3574-25, Test B1, also measures it at 65%. The higher the IFD, the firmer the foam.
The IFD test is the firmness measurement for mattresses, upholstered furniture, automotive seats and polyurethane foam packaging, and the number only means something together with the deflection percentage, the thickness and the specimen size. This guide covers what the acronym means, the step-by-step test per ASTM D3574 (Test B1), ISO 2439 and ABNT NBR 9176, support factor and hysteresis, the difference between IFD and CFD (compression force deflection), why density is not firmness, IFD ranges by application, worked examples with real data, the most common mistakes, the standards with editions checked on October 13, 2026, and how to set up the test on a universal testing machine.
What IFD (ILD) is and what the acronym means
IFD stands for indentation force deflection: the force, in newtons (N) or pounds-force (lbf), that a standardized indenter foot must apply to push into flexible foam to a fixed percentage of its thickness. ILD (indentation load deflection) is the old name for the same test. The Polyurethane Foam Association (PFA), the US foam industry association, records the name change, and ASTM D3574 uses IFD because the result is a force. In stores and on product data sheets both names coexist: "ILD 30" and "IFD 30" mean the same thing.
The test exists because firmness is what the user feels and what the manufacturer needs to specify. Two foams with the same density can be one soft and one firm, depending on the formulation; IFD is the number that tells them apart. That is why it appears in purchasing specifications for slabstock foam, in quality control of mattresses and upholstery, in the approval of automotive seats and in the development of new formulations.
Three points avoid most of the confusion about the number:
- IFD is a force, not a pressure. The result is the load cell reading, in N or lbf. Texts that define IFD as force divided by the foot area (F/A, in pascals) get the quantity wrong: the foot area is fixed by the standard, and the result is reported as a force.
- The number depends on the deflection. "IFD 25%" and "IFD 65%" are different readings of the same foam. When the data sheet says only "IFD 30", the convention in the United States is that it is at 25%, in lbf, on a 4 in (about 100 mm) specimen.
- The number depends on the specimen. Thickness and slab size change the reading, because the foot drags the surrounding foam with it. Two IFD values can only be compared with the same standard, the same thickness and the same specimen size.
IFD in other contexts
The acronym has other uses outside the foam industry. In medicine, IFD means invasive fungal disease, whose consensus definitions are published by the EORTC and the MSGERC (Donnelly et al., Clinical Infectious Diseases, 2020). In this guide, IFD always means the indentation force of flexible foam.
IFD is not hardness of solids
In English, "indentation test" is also the generic name for hardness tests on metals and plastics (Brinell, Vickers, Rockwell, Shore and nanoindentation), in which a small indenter leaves a permanent mark on the surface and the result is a hardness number. IFD is something else: the foot is 203 mm in diameter, the foam recovers its shape after the test and the result is the force at a fixed deflection. Readers looking for hardness of solids will find the topic in the article on hardness testing; for flexible foam, a durometer does not replace the IFD test.
How the IFD test is performed per ASTM D3574 (Test B1)
In Test B1 of ASTM D3574-25, a foam slab resting on a perforated base is flexed twice, rests for 6 min and is then indented by a 323 cm² circular foot to 25% and to 65% of its thickness, with the force read 60 s after each deflection. ASTM D3574 is the US standard of test methods for flexible slab, bonded and molded urethane foams, and it gathers several tests in a single standard: density, IFD, compression set, tensile, tear, ball rebound resilience, fatigue and others, each identified by a letter. IFD is Test B1. The standard is paid and has not been reproduced here; the parameters below are those that agree across the method descriptions published by TestResources and Vilutra and the test documented by Shimadzu (AZoM, 2022). Before setting up a contract test, check them against the purchased standard.
The photo shows a polyurethane foam slab on a perforated steel base, with the circular indenter foot sinking into the center of the slab. The foot is much smaller than the slab, and the foam around it also goes down: this drag is what makes IFD measure firmness as the body feels it, and not only the resistance of the material under the foot.
Test specimen and setup
The test specimen is a slab of the foam or the product itself. The method descriptions call for a slab of at least 380 × 380 × 100 mm (15 × 15 × 4 in), with parallel faces; TestResources notes that 15 × 15 in and 20 × 20 in specimens are the most common. A short definition that circulates online, including in the English Wikipedia entry, mentions 500 × 500 × 100 mm. The difference shows why the report must state the specimen dimensions: the IFD of a larger slab is not the same as that of a smaller slab of the same foam.
The setup has three parts:
- Perforated support plate, larger than the specimen. The holes let air escape from the foam during compression; without them, the air trapped underneath would increase the force reading. In the test documented by Shimadzu, the plate measured 400 × 400 mm, with 6.5 mm holes spaced 20 mm apart.
- Indenter foot, flat and circular, 8 in in diameter (203 mm, area of about 323 cm²), with a slightly rounded edge and a swivel connection to the crosshead, so it seats parallel to the specimen.
- Load cell of low capacity, able to resolve the 4.5 N contact force as well as the forces of hundreds of newtons at 65%. Shimadzu used a 1 kN load cell.
Test B1 step by step
The test is a sequence of deflections and waits, and each one has a physical reason:
- Condition the specimen at the laboratory temperature and humidity defined by the standard, long enough for it to reach equilibrium. Polyurethane foam changes firmness with temperature.
- Pre-flex twice to 75 to 80% of the thickness, at about 250 mm/min, and return. Pre-flexing ruptures the closed membranes left in the cells and erases the recent history of the piece (stacking, transport), so that all specimens start from the same state.
- Let it rest for 6 min without load, so the foam recovers its height.
- Bring the foot into contact at 50 mm/min up to a contact force of 4.5 N (1 lbf) and record the thickness at that point, t₀. This is the reference thickness for all percentages.
- Indent to 25% of t₀ at 50 mm/min, hold the position and read the force after 60 s. This reading is the 25% IFD (F25).
- Indent to 65% of t₀, hold and read the force after 60 s: the 65% IFD (F65).
- Optional: return to 25% and read again, for the return value explained in the section on hysteresis.
- Report t₀, F25, F65, the F65/F25 ratio, the specimen dimensions, the edition of the standard and the conditioning conditions.
The diagram below shows this sequence over time, as a percentage of thickness: the two pre-flexes to 75%, the rest, the contact, the two 60 s holds with the F25 and F65 readings and the optional return to 25%. TestResources estimates that Test B1 takes about 12 min on a 4 in slab, almost half of it in the rest period.
The table summarizes the Test B1 parameters, according to the method descriptions cited above:
| Parameter | Usual value | Why |
|---|---|---|
| Specimen | ≥ 380 × 380 × 100 mm (15 × 15 × 4 in) | larger than the foot, so there is foam around it |
| Indenter foot | Ø 203 mm (8 in), 323 cm² (50 in²) | fixed area, result as a force |
| Pre-flex | 2 times to 75 to 80%, about 250 mm/min | opens cells and erases the history of the piece |
| Rest | 6 min | height recovery |
| Contact and t₀ | 4.5 N (1 lbf), 50 mm/min | reference thickness |
| Readings | 25% and 65% of t₀, after 60 s each | surface and deep support |
Why wait 60 s before reading the force
Polyurethane foam is viscoelastic: held at a fixed deflection, the force it returns drops over time, quickly at first and then more and more slowly. This is stress relaxation. In the test documented by Shimadzu, the force vs. time plots show the force falling during the 60 s holds at 25% and at 65%. If each laboratory read the force at a different moment, the results would not be comparable; the standard fixes the reading time so that everyone reads at the same point of the relaxation. Reading right on reaching 25% gives a number higher than the IFD, and the error varies from foam to foam.
Test B2 and ASTM D5672: variants of the same test
The same standard has an inverse variant, Test B2 (IRGL, indentation residual gage length): instead of reading the force at a fixed deflection, it reads the thickness under fixed forces. In the Shimadzu test, the specimen measured 99.33 mm at 4.5 N, 95.90 mm at 110 N and 70.84 mm at 220 N. ASTM D5672/D5672M-22, in turn, is a rapid production method: it measures the force at a fixed deflection of 25 mm (1 in) on pads at least 75 mm thick, and names Test B1 of D3574 as the referee method when results disagree. It is for screening on the line, not for specification.
ISO 2439 and ABNT NBR 9176: the test outside the USA
Outside the United States, the indentation force of flexible foam follows ISO 2439:2008, which has five methods (A to E), and in Brazil ABNT NBR 9176:2016; the results do not convert to those of ASTM D3574. ISO 2439 is in its 4th edition, from 2008, was confirmed in 2021 and entered systematic review in January 2026, according to the ISO catalog. It applies to open-cell latex, urethane and PVC foams.
The five methods of ISO 2439 differ in deflection and reading time, according to the official abstract of the standard:
- Method A: indentation hardness index, a single reading at 40% after 30 s.
- Method B: characteristic curve, readings at 25%, 40% and 65%, each after 30 s.
- Method C: quick check at 40%, for production control.
- Method D: low hardness index, at 25% after 20 s, for very soft foams.
- Method E: compression deflection coefficient and hysteresis loss.
According to ZwickRoell, the ISO 2439 test specimen measures 380 × 380 × 50 mm (± 2 mm in thickness), is conditioned for at least 16 h per ISO 23529 and is tested on a perforated plate, with a flat indenter 200 to 203 mm in diameter and a rounded edge of about 1 mm radius. For molded parts, such as seat cushions and backrests, the German standard DIN 53579 covers indentation on the finished part, with indenters of other shapes.
In Brazil, ABNT NBR 9176:2016 (Flexible polyurethane foam: determination of indentation force; English title "Indentation force deflection test") is listed as current in the catalog, with confirmations published in 2020 and in January 2025. It belongs to a family of ABNT (Brazilian Association of Technical Standards) standards for flexible polyurethane foam that includes apparent density (NBR 8537), compression set (NBR 8797) and dynamic fatigue (NBR 9177), in which the foam is compressed cyclically and the loss of thickness and of indentation force is measured; the general topic is covered in the article on fatigue testing.
Comparing the three standards shows why the numbers do not mix:
| Item | ASTM D3574-25, Test B1 | ISO 2439:2008 | ABNT NBR 9176:2016 |
|---|---|---|---|
| Deflections | 25% and 65% | 40% (A); 25, 40 and 65% (B); 25% (D) | as per the standard |
| Reading time | 60 s | 30 s (A, B); 20 s (D) | as per the standard |
| Usual specimen thickness | 100 mm (4 in) | 50 mm | as per the standard |
| Result | force, N or lbf | force, N | force, N |
Because deflection, time and thickness change, there is no conversion factor between the ISO 2439 index at 40% and the ASTM 25% IFD, as Vilutra notes. ASTM D3574 itself states in its scope that it knows of no ISO standard equivalent to the whole standard, only to individual methods. A laboratory that receives a US specification tests per D3574; a European specification, per ISO 2439, using the method indicated.
Support factor, 25% return and hysteresis
The support factor is the ratio between the IFD at 65% and the IFD at 25% (SF = F65 / F25). It measures how much the foam stiffens as it is pushed deeper, and according to the PFA it typically falls between 1.5 and 2.6. The same number also appears as compression modulus, sag factor or comfort factor; in Brazil it is called "fator de conforto" (comfort factor). The 25% reading represents the layer that touches the body, the surface feel. The 65% reading represents deep support, once the body weight has already compressed the foam. A foam with a high support factor is soft at the surface and firm underneath, so the body does not "bottom out". The PFA recommends a factor of 2.0 or more for seating.
The physical reason lies in the shape of the force vs. indentation curve. In the first 5 to 10% of deflection, the cell walls bend elastically and the force rises quickly. Then the walls start to buckle and collapse, and the curve enters a plateau: the foam deflects a lot with little increase in force. Near 60%, the collapsed cells press against each other, the foam densifies and the force rises quickly again. The 25% point falls on the plateau; the 65% point, at the start of densification. The ratio between the two shows how well the foam "holds" at depth.
The graph below shows this curve with the values from Example 2, further on: the initial rise, the plateau, the densification, the force drops during the 60 s dwells, the readings of 150 N at 25% and 330 N at 65%, and the return curve, which passes 25% at 120 N. The shaded area between the loading and unloading curves is the hysteresis loop.
Return at 25%. When the indenter foot moves back from 65% to 25% and the force is read again, the value is lower than the F25 on loading. The ratio between the unloading reading and the loading reading, as a percentage, is the return value: in the graph, 120 / 150 = 80%. A high return value indicates a foam that recovers its firmness quickly; a low return value, a foam that is slow to recover, such as viscoelastic (memory) foams. The unloading reading is not the IFD. A retail page cited by AI summaries states that the value read when returning to 25% "is usually the correct ILD"; it is not: the IFD in ASTM D3574 is the reading on loading, after 60 s.
Hysteresis. The unloading curve lies below the loading curve because part of the energy spent compressing the foam is dissipated as heat (internal friction in the polymer and in the air flowing in and out of the cells) and is not returned. Hysteresis loss is the area between the two curves divided by the area under the loading curve, as a percentage; Method E of ISO 2439 standardizes it. In a seat, some hysteresis damps movement; very high hysteresis indicates a foam that absorbs energy and is slow to respond. The concept of returned elastic energy is covered in the article on resilience of materials.
IFD vs. CFD (compression force deflection)
CFD (compression force deflection, or compression load deflection) is the force needed to compress the entire specimen between platens larger than it, reported as stress in kPa (or psi), whereas IFD uses an indenter foot smaller than the specimen and reports force. The difference seems small and changes everything. In the IFD test, the edge of the foot drags the surrounding foam, which also carries load; the result depends on the thickness, the size of the slab and the foam's tensile and shear strength, and it represents the product as the body feels it. In the CFD test, the whole specimen area is compressed uniformly; the result is a material property, nearly independent of specimen size.
The diagram below compares the two setups: in (a), the 203 mm foot indents the center of a 380 × 100 mm slab on a perforated support plate, and the surface around the foot goes down with it; in (b), a 50 × 50 × 25 mm test specimen sits between two platens larger than it, and the compression is uniform.
In ASTM D3574-25, CFD is Test C: the force to compress a specimen of at least 50 × 50 × 25 mm by 50%, divided by the area, in kPa. In the Shimadzu test (AZoM, 2022), the result was 3.578 ± 0.127 kPa at 50%. In ISO, the standard is ISO 3386-1:2025, on compression stress-strain characteristics of low-density flexible foams (up to 250 kg/m³), published in July 2025 to replace the 1986 edition and its 2010 amendment. ISO 3386-1 itself notes that this characteristic differs from the indentation hardness of ISO 2439, which depends on the thickness and tensile properties of the foam.
In practice, IFD is used to specify mattresses, upholstery and seats, where feel matters; CFD is used to specify foams used as materials in joints, seals, gaskets, packaging and cushioning, and it suits small samples that cannot accommodate a 203 mm foot. CFD is a particular case of the compression test, in which both rigid and flexible foams show the plateau typical of cell collapse.
Sponge rubber and gaskets: ASTM D1056
For cellular rubber (sponge or expanded) used in gaskets and seals, the US standard is ASTM D1056-20, which classifies the material by compression-deflection: the force to compress the specimen by 25%, with a platen larger than the specimen, divided by the area, in psi. The standard divides materials into firmness grades: grade 0 from 0 to 2 psi, grade 1 from 2 to 5 psi, grade 2 from 5 to 9 psi, grade 3 from 9 to 13 psi, grade 4 from 13 to 17 psi and grade 5 from 17 to 25 psi. The deflection calculation for a sponge rubber is therefore the CFD calculation: force divided by area at 25% deflection, compared with the range of the specified grade. Example 5 works through this calculation.
Density is not firmness: D28, D33 and D45
Density is the mass of foam per volume, in kg/m³; firmness is the IFD, in N. The two are independent properties: the PFA notes that there are soft high-density foams and firm low-density foams, depending on the formulation. In Brazil, "D33" means a density of 33 kg/m³; in the United States, density is given in lb/ft³ (pcf), and 1 pcf = 16.02 kg/m³. A D33 therefore has about 2.06 pcf, and a US "1.8 density" foam, 28.8 kg/m³.
The confusion comes from the mattress market, which uses density as a synonym for hardness ("D45 is harder than D33"). The proof that it is not lies in the mattresses analyzed by the PFA in its IN-TOUCH bulletin (version 5.1, revised in 2018), which gives the density and the 25% IFD measured in the core of four mattresses. The graph below sorts the four cores by increasing density, from 24 to 42 kg/m³: the IFD drops from 191 N to 120 N. The least dense foam is the firmest. In the hatched bar, the topper of mattress D, at 32 kg/m³, has only 44 N: dense for a topper and very soft.
What density does, then, is determine durability. More density means more polymer per volume, thicker cell walls and less loss of height and firmness with use. According to the PFA, foams of 2.0 pcf (32 kg/m³) or more lose less than 5% of their thickness in service, and a 2.0 pcf foam loses 15 to 20% of its firmness after 75,000 cycles in the constant force pounding test. Density also tends to raise the support factor (PFA, glossary). In a specification, the two numbers go together: density for durability, IFD for firmness.
In Brazil, Inmetro's mattress certification (Ordinance 35/2021, amended by Ordinance 579/2023, Brazilian regulatory requirements) sets minimum densities: 20 kg/m³ for foam mattresses and mats in general, 28 kg/m³ for auxiliary mattresses, combined box spring mattresses and hybrid mattresses, and 18 or 20 kg/m³ for children's mattresses, depending on length (up to 1,500 mm or above). The "density by weight and height" tables circulating in Brazilian retail are market recommendations, not standard requirements: no standard says how many kilograms a D50 "supports".
The table converts the most common densities between Brazilian and US usage (Biopdi calculation, 1 pcf = 16.018 kg/m³):
| Brazil | kg/m³ | pcf (lb/ft³) |
|---|---|---|
| D20 | 20 | 1.25 |
| D28 | 28 | 1.75 |
| D33 | 33 | 2.06 |
| D40 | 40 | 2.50 |
| D45 | 45 | 2.81 |
| D50 | 50 | 3.12 |
Typical IFD values by application
Most flexible polyurethane foams have a 25% IFD between about 15 and 70 lbf (67 to 311 N), measured on a 4 in sample. The ranges below are market references published by manufacturers and distributors (TestResources, The Foam Factory and Foamonline), not standard requirements; they give an order of magnitude for the number on a data sheet, not a basis for accepting a lot. The conversion is by Biopdi, with 1 lbf = 4.448 N.
| IFD at 25% (lbf) | IFD at 25% (N) | Typical use | Source |
|---|---|---|---|
| below 20 | below 89 | comfort layers, toppers, seat backs | The Foam Factory |
| 20 to 40 | 89 to 178 | comfort with support, mattresses and upholstery | The Foam Factory |
| 35 | 156 | medium-firm mattress | TestResources |
| 45 | 200 | very firm mattress | TestResources |
| 50 or more | 222 or more | firm seats, automotive seats | The Foam Factory |
| 15 to 70 | 67 to 311 | range of most foams | Foamonline |
To read a US data sheet, converting from lbf to N is straightforward. The table below gives the round values most often quoted in stores and data sheets:
| lbf | N | lbf | N |
|---|---|---|---|
| 10 | 44.5 | 44 | 195.7 |
| 20 | 89.0 | 50 | 222.4 |
| 25 | 111.2 | 60 | 266.9 |
| 30 | 133.4 | 70 | 311.4 |
| 35 | 155.7 | 80 | 355.9 |
"44 ILD" on a mattress therefore means about 196 N to indent a 4 in sample by 25%: a firm foam, close to what TestResources associates with a very firm mattress. The number is only meaningful if the data sheet uses the same reference (25%, 4 in sample, ASTM D3574); if the sheet follows ISO 2439 at 40%, the value is different.
Worked examples
The examples below use published data (1 and 3) and illustrative values consistent with industry ranges (2, 4 and 5). The calculations are by Biopdi, with 1 lbf = 4.448 N, 1 pcf = 16.018 kg/m³, 1 psi = 6.895 kPa and 50 in² = 322.6 cm².
1. Real test, ASTM D3574 B1 (Shimadzu, AZoM, 2022). Readings: F25 = 156.652 N and F65 = 317.683 N. Support factor: SF = 317.683 / 156.652 = 2.03, within the PFA range of 1.5 to 2.6 and above 2.0, the value it recommends for seating. In lbf: 156.652 / 4.448 = 35.2 lbf and 317.683 / 4.448 = 71.4 lbf. A 25% IFD of 35 lbf is what TestResources associates with a medium-firm mattress. Just for order of magnitude, the average pressure under the foot is 156.652 N / 0.03226 m² = 4,856 Pa, or 4.86 kPa; the test result is still the force.
2. Complete test with measured t₀ (illustrative values). A nominal 380 × 380 × 100 mm slab measured t₀ = 98.8 mm under 4.5 N. Deflection at 25%: d = 0.25 × 98.8 = 24.7 mm, leaving the foam at 74.1 mm. Deflection at 65%: d = 0.65 × 98.8 = 64.2 mm, leaving the foam at 34.6 mm. Readings after 60 s: F25 = 150 N and F65 = 330 N. SF = 330 / 150 = 2.20, suitable for seating. On the return to 25%, the reading was 120 N: return value = 120 / 150 = 80%. In lbf: 150 / 4.448 = 33.7 lbf and 330 / 4.448 = 74.2 lbf. Had the operator used the nominal thickness of 100 mm, the 25% target would have been 25.0 mm, which is 25.3% of the actual thickness (25.0 / 98.8): the force read would no longer be the 25% IFD.
3. Density vs. firmness (PFA, IN-TOUCH, rev. 2018). Core of mattress A: 1.5 pcf × 16.018 = 24.0 kg/m³ and 43 lbf × 4.448 = 191 N. Core of mattress D: 2.6 pcf × 16.018 = 41.6 kg/m³ and 27 lbf × 4.448 = 120 N. The 24 kg/m³ foam is 191 / 120 = 1.59 times firmer than the 41.6 kg/m³ foam. Cores B (1.8 pcf, 28.8 kg/m³, 36 lbf, 160 N) and C (2.4 pcf, 38.4 kg/m³, 30 lbf, 133 N) follow the same trend. The topper of mattress D, at 2.0 pcf (32.0 kg/m³) and 10 lbf (44.5 N), is denser than core A and four times softer.
4. Retail data sheets. "44 ILD" = 44 × 4.448 = 195.7 N; "35 ILD" = 155.7 N; "45 ILD" = 200.2 N, always at 25% on a 4 in sample. A "1.8 density, 35 ILD" foam has 1.8 × 16.018 = 28.8 kg/m³ (close to a Brazilian D28) and an IFD of 156 N.
5. Sponge rubber, ASTM D1056-20 (illustrative values). A test specimen with an area of 6.45 cm² (1 in²) required 7 lbf for 25% deflection. Compression-deflection: 7 lbf / 1 in² = 7 psi = 7 × 6.895 = 48.3 kPa. The value falls in grade 2 (5 to 9 psi). Note the difference from IFD: here the result is force per area, because the platen covers the entire specimen.
Common mistakes in the IFD test
The mistakes that most distort the IFD, or its interpretation:
- Measuring t₀ without the contact force, or using the nominal thickness. The deflection percentage comes out wrong, as in Example 2.
- Skipping the preflex or the rest period. The first compression of a new foam gives a higher force than the following ones; without preflexing, the result is not comparable.
- Reading before 60 s, or reading the peak on reaching 25%. Relaxation has not yet occurred, and the number comes out high.
- Calling the unloading reading the IFD. The IFD is the loading reading; the unloading reading measures the return value.
- Defining IFD as pressure (F/A). The result is a force, in N or lbf.
- Comparing samples of different thickness or size. The foot drags the surrounding foam, and a larger or thicker slab gives a different number.
- Mixing ASTM and ISO. IFD at 25% and 60 s does not convert into an ISO index at 40% and 30 s.
- Not conditioning the sample. The firmness of polyurethane foam changes with temperature and humidity.
- Supporting the sample on a plate without holes. Air trapped underneath increases the force.
- Ignoring the position of the foot. TestResources notes that the IFD varies with the location on the part and between lots of the same production run; the foot goes at the center, and the report states where the sample was taken from.
- Using density as a measure of firmness. D45 is not "harder" than D33 by definition; firmness is the IFD.
- Citing an old edition. The current edition of ASTM D3574 is -25; -17 has been superseded.
Applications of the indentation force deflection test
IFD is the firmness test for every product in which a body rests on flexible foam. In mattresses and upholstered furniture, it defines surface feel (IFD at 25%) and support (support factor), while density, with the minimums set by Inmetro (Brazil's national metrology and certification institute), defines durability. In automotive seating, IFD specifies seat and backrest cushions; NHTSA, the US road safety agency, used IFD to evaluate the foams of the child restraint test bench in FMVSS 213 (report DOT HS 813 129, 2021), including the effect of temperature and humidity. In protective packaging, foam firmness decides how far the foam sinks under the product's weight before it cushions an impact.
When the foam stops being the product and becomes a material, as in joints, seals and gaskets, the reference test becomes CFD, explained above. And when the interest is durability, the IFD measurement is repeated before and after a fatigue test to measure the loss of firmness with use.
Foam indentation test standards
The standards for the flexible foam indentation test are ASTM D3574-25 (Test B1) in the United States, ISO 2439:2008 in the rest of the world and ABNT NBR 9176:2016 in Brazil; for CFD, ASTM D3574-25 (Test C) and ISO 3386-1:2025. The table gives the edition and status as checked in the ASTM store, the ISO catalog and the ABNT catalog on October 13, 2026. ASTM D3574-25 was published in December 2025 and replaced "Urethane" with "Polyurethane" in the title.
| Standard and edition | Subject | Status on October 13, 2026 |
|---|---|---|
| ASTM D3574-25 | flexible polyurethane foams; IFD (B1), IRGL (B2), CFD (C) and other tests | active; replaced D3574-17 |
| ISO 2439:2008 | indentation hardness of flexible foam, methods A to E | 4th edition, confirmed in 2021, under systematic review since January 2026 |
| ABNT NBR 9176:2016 | flexible polyurethane foam, indentation force | current, confirmed in January 2025 |
| ISO 3386-1:2025 | stress × strain in compression, flexible foam up to 250 kg/m³ | 3rd edition, published in July 2025 |
| ASTM D5672/D5672M-22 | indentation force at a fixed 25 mm (1 in) deflection, production screening | -22 edition, current |
| ASTM D1056-20 | cellular rubber for gaskets; compression-deflection at 25%, grades 0 to 5 | -20 edition, current |
| DIN 53579 | indentation of molded foam parts | edition not checked |
| ISO 23529 | conditioning of rubber and foam test specimens | edition not checked |
The ABNT (Brazilian) standards for apparent density (NBR 8537), compression set (NBR 8797) and dynamic fatigue (NBR 9177) of flexible polyurethane foam complete the characterization; the edition and status of each should be checked in the ABNT catalog before use.
How to set up the IFD test on a universal testing machine
The IFD test is performed on a universal testing machine in compression, with the standard indenter foot attached to the load cell on the crosshead and the perforated support plate on the base, in place of the compression platens. The setup requires six things from the machine: controlled speeds of 50 and 250 mm/min; position held during the 60 s dwells, with force recorded; a low-capacity load cell that reads with resolution both the 4.5 N contact force and the hundreds of newtons at 65%; travel for 80% of the thickness plus the setup clearance; software that chains preflexes, rest, dwells and the support factor calculation; and space between the columns for the support plate, which is about 400 mm.
This last point decides the frame. A 380 mm plate with the foot at the center places the test axis 190 mm from the edge of the sample. In Biopdi's M2 line, the single-column and extended single-column (MBIOI-10-M2) machines have a test space width of 150 mm and do not fit the plate; the smallest frame that fits it by test space width is the dual-column MBIOII-50-M2, with 425 mm between columns, 5 to 50 kN, speed from 0.02 to 500 mm/min and 1,300 mm of travel. CFD on small specimens (Test C, 50 × 50 × 25 mm) also fits on the single-column machines, which the Biopdi website recommends for foams, using the compression platens. The panel below shows the machine, the role of each part in the IFD test and the compression platens of the line.
The universal testing machine in the foam indentation test
The M2 line has single-column models, from 200 N to 10 kN, and dual-column models, from 5 kN to 1,000 kN. Each part of the machine has a role in the test:
- Moving crosshead lowers the foot at 250 and 50 mm/min and holds position
- Load cell of low capacity, from 4.5 N to hundreds of newtons
- Indenter foot supplied by Biopdi, in place of the platens; perforated support plate per the standard
- Deflection from the crosshead, as a percentage of t₀
- BioLab software saved method with dwells and custom calculations
- Control panel and emergency stop positions and stops the test
M2 line compression fixtures
The platens below are mounted on the same machine and are used for CFD and foam compression:
In the BioLab software, a Method stores the complete configuration of a test, with parameters, stops and custom calculations, and is reapplied the same way by any operator; this is where the Test B1 sequence and the F65/F25 ratio are configured. The load cell is chosen according to the expected force: an IFD at 65% of a few hundred newtons and the 4.5 N contact force call for the smallest load cell compatible with the frame, defined in the quote. Force calibration and the load cell class in the low range are explained in universal testing machine calibration.
| Application | Range and test space width | Biopdi model |
|---|---|---|
| IFD on a 380 × 380 mm slab (ASTM D3574 B1, ISO 2439) | 5 to 50 kN; 425 mm test space width | MBIOII-50-M2 dual-column |
| CFD on a small specimen (ASTM D3574 C, ISO 3386-1) | 2 to 10 kN; 150 mm test space width | MBIOI-10-M2 and MBIOI-10-M2 extended column |
The same machine performs the tensile, tear and compression tests of the foam, which complete the technical data sheet. To compare frames and capacities, see how to choose a universal testing machine and the Biopdi universal testing machine page.
Other tests and technical references
Tests performed on the Biopdi universal testing machine
Related technical references
ASTM D790: three-point flexural test of plastics (procedures A and B, support span and calculations)
ASTM D790-25 measures the flexural strength and flexural modulus of plastics using a bar on two supports loaded at midspan. The support span is 16 times the thickness. The strain rate is 1%/min (procedure A) or 10%/min (procedure B), and the test runs until the specimen breaks or reaches 5% strain. Covers the crosshead speed R = ZL²/6d, the formulas, the large-deflection correction, a comparison with ISO 178 and D6272, and worked examples.
Coefficient of friction: what it is, formula, table and test (ASTM D1894)
The coefficient of friction (μ) is the ratio of the friction force to the normal force between two surfaces. See the formula, static and kinetic friction, the inclined plane, a table of values, solved exercises and film testing per ASTM D1894 and ISO 8295.
Hardness test: types, procedure and standards (Brinell, Rockwell, Vickers and Shore)
A hardness test presses a standardized indenter into the part under controlled force and dwell time, then measures the indentation or its depth. See the types, the step-by-step procedure for each method, minimum thickness and spacing, hardness tester verification and standards.
Frequently asked questions
What does IFD stand for?
IFD stands for Indentation Force Deflection: the force needed for a 323 cm² (8 in diameter) indenter foot to depress a flexible foam by a percentage of its thickness, 25% and 65% in Test B1 of ASTM D3574-25. It is the standard measure of foam firmness. In medicine, the same acronym means invasive fungal disease.
Are IFD and ILD the same thing?
Yes. ILD (Indentation Load Deflection) is the old name of the same test; ASTM D3574 and the PFA use IFD, because the result is a force. "ILD 30" and "IFD 30" indicate the same firmness.
What does 44 ILD mean on a mattress?
That the foam needed about 44 lbf (196 N) to be depressed 25% on a 4 in (about 100 mm) sample with the 323 cm² foot. It is a firm foam: TestResources associates 35 lbf with a medium-firm mattress and 45 lbf with a very firm mattress.
What is compression force deflection (CFD)?
It is the force needed to compress the entire sample between platens larger than it, divided by the area and reported in kPa or psi. In ASTM D3574-25 it is Test C, at 50% on a 50 × 50 × 25 mm specimen; in ISO, it is ISO 3386-1:2025. CFD measures the material; IFD, with a foot smaller than the sample, measures the product as the body feels it.
What is a load deflection test?
It is any test that records force as a function of deflection (displacement). In foam, the load deflection test is IFD or CFD; in beams and structural parts, deflection is measured in the flexural test.
D33 or D45: which foam is firmer?
Density does not answer that: firmness is IFD, and the PFA documents less dense foams that are firmer than denser ones. D45 has more polymer per volume and tends to last longer and lose less height with use; whether it is firmer than D33, only IFD tells.
What is the difference between density 40 and 50 foam?
First, check the unit: 40 kg/m³ equals 2.5 pcf and 50 kg/m³ equals 3.1 pcf. More density means more durability and, in general, a higher support factor, not more firmness. For firmness, compare IFD at 25% measured by the same standard.
Can an ISO 2439 result be converted to ASTM IFD?
No. ISO 2439 (method A) reads at 40% after 30 s on a 50 mm sample; ASTM D3574 B1 reads at 25% and 65% after 60 s on a 100 mm sample. With different deflection, time and thickness, there is no reliable conversion factor.
What is the Brazilian standard for the indentation force test?
ABNT NBR 9176:2016, Flexible polyurethane foam: determination of indentation force, current in the ABNT catalog on October 13, 2026, with a confirmation published in January 2025.
What machine performs the IFD test?
A universal testing machine in compression, with a low-capacity load cell, the 203 mm indenter foot and the perforated support plate of the standard, and software that performs the 60 s dwells and calculates the support factor. For a 380 mm slab, the machine needs a test space width larger than the support plate, about 400 mm.
How long should you wait before reading the force?
60 s at each deflection in Test B1 of ASTM D3574; 30 s in methods A and B of ISO 2439 and 20 s in method D. The wait exists because the foam's force drops over time (relaxation), and all laboratories need to read at the same point.