If you have read a detailed mattress spec sheet, you may have spotted "ILD" next to the foam layers and wondered what it means. It is a genuine engineering measure of foam firmness, but on its own it tells you less than most people assume.
This short guide explains what ILD measures, how it differs from density and overall firmness, and how to use it sensibly when comparing mattresses.
What ILD Measures
ILD stands for Indentation Load Deflection. It measures the force needed to compress a sample of foam by a set amount, usually pressing a round plate 25 percent into the foam's thickness.
A higher ILD means the foam resists compression more, so it feels firmer. A lower ILD means it gives way more easily, so it feels softer. In short, ILD is a firmness number for a single foam layer.
How ILD Is Tested
ILD is measured in a lab on a standard foam sample. A circular plate presses into the foam and the machine records the force, in newtons or pounds, required to reach the set compression depth.
Because it is measured on an isolated sample, ILD describes that one foam layer, not how the finished mattress feels once all the layers and springs work together.
ILD vs Density vs Firmness
This is where shoppers get tripped up. These three terms describe different things.
| Term | Measures | Tells you |
|---|---|---|
| ILD | Force to compress the foam | How firm that foam layer feels |
| Density | Foam material per cubic metre | Durability and quality |
| Overall firmness | How the whole mattress feels | Comfort for your body |
A common myth is that high density means firm. It does not. Density and ILD are independent; a dense foam can have a low ILD and feel soft. For the durability side, see our guide on memory foam density.
Why a Single ILD Number Cannot Tell You How a Mattress Feels
A mattress is a stack of layers over a support core. The comfort foam might have a low ILD for softness, sitting on a higher-ILD support foam, over zoned pocket springs.
What you feel is the combined result, plus your own body weight and sleep position. A lighter person will perceive the same mattress as firmer than a heavier person will. That is why ILD is useful for comparing similar foams but cannot replace lying on the mattress, ideally over a sleep trial. Our mattress firmness guide explains how to match feel to your body.
Using ILD as One Signal Among Several
Treat ILD as a helpful detail, not a deciding factor.
- Use it to compare the relative softness of comfort layers between two similar mattresses.
- Pair it with density to understand both feel and durability.
- Always weight your own test of the mattress over any single spec.
Frequently Asked Questions
Is a higher ILD firmer or softer?
Higher ILD is firmer. It means more force is needed to compress the foam, so it resists your body weight more and feels harder. Lower ILD feels softer.
What ILD is good for side sleepers?
Side sleepers generally benefit from a softer comfort layer to cushion the shoulder and hip, which means a lower ILD near the surface, ideally over a supportive core. Rather than chasing a number, focus on whether the mattress relieves pressure at your shoulders and hips.
Is ILD the same as density?
No. ILD measures firmness, density measures how much foam material is packed in. They are independent, so a dense foam can feel soft and a low-density foam can feel firm.
How do I know a mattress's firmness?
The most reliable way is to lie on it in your usual sleep position, ideally with a home sleep trial. Spec numbers like ILD help compare options, but perceived firmness depends on the whole construction and your own body weight.
