IFD vs ILD: What the Difference Means for Foam Buyers
Foam spec sheets, mattress listings and supplier quotes use two abbreviations for firmness: IFD and ILD. Buyers often assume they’re different tests and try to convert one into the other. They aren’t. This article explains IFD vs ILD, how the measurement is actually made under ASTM D3574, what the 25% and 65% values and the support factor tell you, and the mistakes that make two IFD numbers impossible to compare. For the wider picture of foam testing, see our foam testing guide.
IFD vs ILD: one test, two names
ILD stands for indentation load deflection and IFD for indentation force deflection. Both describe the same thing: the force needed to push a flat circular indentor into a foam sample by a set percentage of its thickness. ILD is the older term. The current ASTM D3574 standard uses IFD, because the result is a force, not a load in the old sense of the word.
In practice, the bedding and furniture trades still use both, often in the same conversation. When someone quotes “an ILD of 30” without saying more, they usually mean the 25% IFD value in pounds-force. So the real question isn’t IFD or ILD, but which indentation level, which test method and which sample thickness the number came from.
How IFD is measured under ASTM D3574 Test B1
Test B1 is the IFD method most US foam specifications refer to. The main steps are:
- Prepare the sample. The standard specimen is 380 × 380 mm (15 × 15 in) and 100 mm (4 in) thick, conditioned before testing.
- Preflex it. The indentor compresses the sample to 75-80% of its thickness, usually twice, to remove the stiffness left by manufacturing and storage.
- Let it rest. The sample stands for about six minutes before the measured run.
- Read the 25% value. The indentor compresses the foam to 25% of its thickness, holds for about 60 seconds, and the force is recorded.
- Read the 65% value. Compression continues to 65%, holds for about 60 seconds, and the second force is recorded.
The indentor is a flat disc with an area of about 323 cm² (50 in²). Results are reported in newtons or pounds-force.
Reading the numbers: 25%, 65% and support factor
| Value | What it tells you | Where it matters |
|---|---|---|
| 25% IFD | Surface firmness, the first feel when you sit or lie down | Comfort layers, cushion feel |
| 65% IFD | Deep support under full body weight | Support cores, seat bases |
| Support factor (65% ÷ 25%) | How much harder the foam pushes back as you sink deeper | Comparing foams with similar surface feel |
Two foams with the same 25% IFD can feel very different once you put your full weight on them. A higher support factor means the foam stays soft at the surface but firms up progressively, which is why high resilience (HR) foams are prized in mattresses and seating. A low support factor means the foam bottoms out more easily.
IFD and ISO 2439 hardness
Outside North America, many buyers specify hardness to ISO 2439 instead. Its common method A reads the force at 40% indentation after a 30 second hold, on a thinner sample than ASTM uses. The principle is the same, but the indentation level, hold time and sample thickness all differ, so an ISO value and an IFD value for the same foam won’t match. There’s no reliable conversion factor between them. If a supplier quotes ISO and your specification is written in IFD, ask for a retest to the method you specify. The full cross-reference of ASTM D3574 and ISO methods is in ASTM D3574 vs ISO.
Hardness is also only one of three numbers that describe a foam. How it relates to density and resilience is covered in foam density vs hardness vs resilience.
Common mistakes when comparing IFD values
- Different sample thickness. A thinner sample gives a different IFD than the standard 100 mm specimen, even for the same foam.
- Skipping the preflex. An unflexed sample reads firmer than it really is.
- Testing before conditioning. Temperature and humidity change firmness, especially in viscoelastic foams.
- Mixing ISO and ASTM values. A 40% ISO reading and a 25% IFD reading are not the same measurement.
- Reading one value only. A 25% IFD without the 65% value says nothing about deep support.
What IFD doesn’t tell you
IFD describes firmness on the day of the test. It doesn’t tell you how much material is in the foam; that’s density, measured as described in foam density test standards. Nor does it predict how the foam will age. A foam can start at the right IFD and lose a large share of it within months of use.
Frequently asked questions
Is IFD the same as ILD?
Yes. They’re two names for the same indentation measurement. ILD is the older term; ASTM D3574 uses IFD.
Is a higher IFD always firmer?
At the same indentation level and sample thickness, yes. Comparing a 25% value with a 65% value, or values from different sample thicknesses, gives a misleading answer.
Can I convert ISO 2439 hardness to IFD?
Not reliably. The methods use different indentation levels, hold times and sample sizes. The safe approach is to test both foams to the same method.
Does IFD drop over time?
Yes. Repeated loading softens foam, and the loss is measured with a foam fatigue test, which reports hardness and thickness loss after thousands of cycles.