Foam density vs ILD is the pair of numbers that ends more cushion arguments than any other, mostly because buyers send one of them and assume it covers both. A US buyer writes "35 density" into the tech pack. Your factory reads it as 35 kg/m³ and quotes. The buyer was thinking of firmness. Both sides sign off believing the cushion is agreed, and nobody finds out otherwise until the first container of sofas lands and the seats feel like a park bench.
These two numbers are not two versions of the same thing. One predicts how long the cushion lasts. The other predicts how it feels the day it is unwrapped. Below are the six beliefs that cause the most rework in furniture export, what is actually true, and the four-line foam spec that stops the argument before the order.
Foam density vs ILD: what each number actually measures
Foam density is mass per unit volume — how much polyurethane is in the block — measured in kilograms per cubic metre (kg/m³) or pounds per cubic foot (pcf). ILD (indentation load deflection) is a force: the pounds needed to compress a foam sample by a stated percentage of its height. Density is a material property. ILD is a behaviour under load. Neither one lets you calculate the other.
The Polyurethane Foam Association states it flatly: "Firmness is independent of foam density, although it is often thought that higher density foams are firmer."
That single line is the whole problem, so start there.
Myth 1: Higher density foam is firmer foam
It is not. It is entirely possible to make a high-density foam that feels soft and a low-density foam that feels firm — the chemistry that sets cell structure and the chemistry that sets load-bearing are different levers on the same machine.
What density does predict is staying power. More material per cubic metre means more cell walls sharing the load, which is why density tracks with resistance to sagging and fatigue over years of sitting. So when a buyer asks "is this good foam?", density answers for how long, and ILD answers how it feels now.
Say it to the buyer in that order and half the email thread disappears.
Myth 2: "30 density" is a spec
"30 density" is a number without a unit, and the two units in daily use are 16 times apart. One pound per cubic foot equals 16.018 kg/m³. A spec line that omits the unit is not a spec — it is a coin flip.
| kg/m³ | pcf (lb/ft³) |
|---|---|
| 20 | 1.25 |
| 25 | 1.56 |
| 30 | 1.87 |
| 35 | 2.19 |
| 40 | 2.50 |
| 45 | 2.81 |
| 50 | 3.12 |
Practical reading rule, and this is trade practice rather than a written standard: seating-grade flexible foam is quoted in single digits of pcf, so a bare "30" in a tech pack from a metric-working supplier is almost always kg/m³, and a bare "30" from a US buyer is at least as likely to be an ILD value in pounds. Do not guess which. Write both units on every quotation line and let the number defend itself.
Myth 3: ILD and IFD are two different tests
They are the same measurement under two names. ILD is the older term; IFD (indentation force deflection) is the term used in the current ASTM method, and the trade uses both interchangeably.
The test is ASTM D3574, Test B1. The Polyurethane Foam Association describes it as "the force (in pounds) required to compress a 50 square inch circular indentor foot into a four inch thick sample… to a stated percentage of the sample's initial height."
Three details in that sentence decide whether two labs agree:
- The sample is four inches thick. An ILD quoted on a 2-inch pad is not comparable to one quoted on a 4-inch pad.
- The indentor is 50 square inches. A different foot gives a different number.
- The deflection percentage must be stated. "ILD 30" with no percentage is as incomplete as "30 density".
ASTM D3574 is a suite, not a single test — Test A is apparent density, B1 and B2 are indentation, C is compression force deflection, D is compression set, E is tensile, F is tear, G is air flow, H is ball rebound. When a buyer asks for "the foam report", they usually want A, B1, D and H on one page.
Myth 4: One ILD number describes the cushion
The 25% IFD tells you how the cushion greets you. It says nothing about what happens when a 95 kg adult settles in for a film.
That second question is answered by support factor, the ratio of the 65% IFD to the 25% IFD. The Polyurethane Foam Association puts the usable range at about 1.5 to 2.6, and notes that a higher number means better support — the sitter does not "bottom out".
Run the arithmetic on two cushions that feel identical at first touch:
| Cushion | 25% IFD | Support factor | Implied 65% IFD |
|---|---|---|---|
| A | 30 lb | 1.7 | 51 lb |
| B | 30 lb | 2.4 | 72 lb |
Same handshake, completely different sit. Cushion B still has 40% more resistance left at deep compression. If your buyer's complaint is "it feels fine in the showroom and dead after an hour", the number that failed is support factor, not density.
Myth 5: Dense foam cannot sag
Density improves the odds. It does not prove the outcome. The property that actually describes sag is compression set, defined by the association as "a permanent loss of initial height of an FPF sample after compression due to a bending or a collapse of the cell framework", and it is measured by ASTM D3574 Test D.
A high-density foam with poor compression set will still lose height. A buyer who has been burned once will ask for the Test D result, not for a bigger density number — and if you volunteer it before they ask, you have quietly moved yourself into a different supplier tier.
Myth 6: "HR foam" is a marketing label
High resilience is a defined product class, not a sales adjective. The association's glossary is explicit that HR flexible polyurethane foams "have a high support factor and greater surface resilience than conventional FPFs and are defined in ASTM D3770."
Resilience itself is measured by dropping a steel ball onto the foam and recording the rebound percentage, and the association puts the practical spread at about 20% rebound to as high as 80%. If a supplier writes "HR foam" with no resilience figure and no support factor, treat it as a claim, not a spec — because that is exactly what a competent buyer will do to you.
The foam density vs ILD spec line that stops the argument
Four lines plus a market rule. This is the block to paste into a quotation, a tech pack reply, or a spec sheet.
Foam spec block
- Density: value in kg/m³ and pcf, tested to ASTM D3574 Test A
- Firmness: 25% IFD value in pounds (or newtons), sample thickness stated, tested to ASTM D3574 Test B1
- Support factor: 65% IFD ÷ 25% IFD, stated as a ratio
- Compression set: percentage, tested to ASTM D3574 Test D
- Flammability: the destination market's rule, named by number, not by the word "compliant"
That last line is where furniture exporters lose whole orders. Upholstered furniture sold into the United States has to meet the smolder-resistance requirements of California Technical Bulletin 117-2013, which the federal government adopted for the whole country in 16 CFR Part 1640. The United Kingdom runs a separate and stricter regime under the Furniture and Furnishings (Fire) (Safety) Regulations 1988. A foam spec with perfect density and ILD and no flammability line is not a finished spec in either market.
Where the four numbers actually have to end up
Here is the part most factories skip. The numbers live in a tech pack the buyer's merchandiser reads once, and then vanish. The people who make the reorder decision — the buyer's sales team, the retail category manager, the end customer reading a listing — never see them.
The fix is not "write a better tech pack." It is putting the cushion cross-section on the image, with each layer measured and labelled where the eye already goes: seat height, cushion thickness, core versus wrap, density and IFD beside the layer they belong to. The tool that does this well locks the measured dimension onto the photo — you snap to the real edge of the object and the label carries the true number, then export at whatever spec size the platform or catalogue wants. This is the difference that matters when you compare it with an AI image generator: a generated picture can put a confident "10 cm" on a cushion that is 8 cm, because it is drawing a plausible label rather than measuring anything. On a spec diagram, a plausible number is a defect.
Two places that pays off immediately: the furniture spec diagram you attach to a quotation, and the listing image where you have already committed to standard sofa dimensions. Once the foam block is on the image, the "what density is it?" email stops arriving.
Pre-quote foam check
- Every density value carries a unit — kg/m³ and pcf both printed
- Every IFD value states the deflection percentage and the sample thickness
- Support factor is stated, not implied by the 25% number
- Compression set result is on the same page as density
- Flammability rule is named by number for the destination market
- The same four numbers appear on the spec image, not only in the tech pack
- Fabric specs travel with the foam specs — see upholstery fabric specifications
- Packed and assembled sizes agreed alongside the cushion spec — see sofa dimensions for export
FAQ
Is higher density foam firmer?
No. Firmness is independent of density, and the Polyurethane Foam Association says so directly. You can have a dense soft foam and a light firm one. Density predicts durability; ILD predicts how firm it feels.
What does ILD mean in foam?
ILD means indentation load deflection: the force in pounds needed to compress a foam sample by a stated percentage of its height, using a 50 square inch indentor foot on a four-inch sample under ASTM D3574 Test B1. Modern documents often call the same measurement IFD.
What is the difference between ILD and IFD?
There is no difference in the measurement. ILD is the older name, IFD is the name used in the current ASTM method, and the trade uses both. If a document mixes them, it is a naming habit, not a different test.
What is a good foam density for a sofa seat cushion?
There is no single correct number, and any supplier who gives you one without asking about the market is guessing. Specify it as a set: density for expected life, 25% IFD for the feel the buyer signed off on, support factor so it does not bottom out, compression set so it does not flatten, and the destination market's flammability rule. A buyer who receives that set stops negotiating on the density number alone.
How do I show foam specs to a buyer who never opens the tech pack?
Put them on the image. A labelled cushion cross-section with real measured thicknesses beside each density and IFD value gets read by people who will never open an attachment — and because a dimension annotation tool measures against the actual object rather than generating a caption, the number on the picture is the number in the spec. That is what makes the image usable as a quotation attachment instead of decoration.
Sources & References
- Polyurethane Foam Association — Foam Performance: density, IFD, support factor and resilience
- Polyurethane Foam Association — Glossary of flexible polyurethane foam terms
- ASTM D3574 — Standard Test Methods for Flexible Cellular Materials: Slab, Bonded, and Molded Urethane Foams
- California Bureau of Household Goods and Services — Technical Bulletin 117-2013, smolder resistance of upholstered furniture materials
- eCFR — 16 CFR Part 1640, Standard for the Flammability of Upholstered Furniture
- UK legislation — The Furniture and Furnishings (Fire) (Safety) Regulations 1988