How Mattress Firmness Is Measured

Mattress firmness is usually described with a number on a 1-to-10 scale, but that number is not standardized across manufacturers. Two mattresses both labeled a '6' can compress quite differently under the same weight, because the underlying measurement is a manufacturer-specific test, not a regulated industry standard.

Understanding what firmness rating actually measures — and what it leaves out — explains why the same number means different things depending on which company assigned it.

How a Firmness Rating Is Actually Produced

The most common method used across the mattress industry is an indentation load deflection test, which measures how far a flat surface presses into the mattress under a specified amount of force, expressed as a percentage of compression. A firmer mattress compresses less under the same force; a softer one compresses more.

Manufacturers then convert that compression measurement into a 1-to-10 firmness scale for marketing purposes. Because no regulatory body defines exactly how that conversion should work, each company sets its own scale, informed by its own product line and its own internal testing panel's subjective feedback rather than a single external reference point.

Some manufacturers apply the indentation test at a single point in the center of the mattress, while others average readings across several points on the surface, which is a further methodological difference that can shift the resulting number even for physically similar mattresses, since neither approach is required by any external standard-setting body.

What Actually Determines How a Mattress Feels

Several layered components combine to produce a mattress's felt firmness. The core support layer — either coils or a dense foam base — determines the mattress's overall structural resistance. The comfort layers sitting above it, often memory foam, latex, or a pillow-top material, determine how the surface responds to initial pressure before the firmer support layer engages.

Body weight and sleep position also interact with these layers differently for different people, which is part of why the same physical mattress can feel meaningfully different to two people of different weights or who sleep in different positions.

Where the Number Breaks Down

Because firmness scales are not standardized, a rating is only reliably comparable within a single manufacturer's own product line, not across brands. A '7' from one company may compress closer to what another company calls an '8' under the same test conditions.

Firmness is also frequently conflated with support, but the two are mechanically distinct. Support refers to whether the spine stays aligned during sleep, which depends on the underlying structural layer, while firmness refers only to how much the surface compresses under pressure. A mattress can be soft to the touch and still structurally supportive, or firm and still poorly aligned for a particular body type.

Firmness also changes over the life of a mattress in a way the original rating never accounts for. Foam layers gradually lose some of their compression resistance through years of repeated use, which means the firmness number printed at the time of purchase describes the mattress only in its original, unworn state, not its condition several years later.

What Independent Testing Shows

Independent consumer-testing organizations that measure mattresses with standardized instruments, rather than relying on manufacturer-supplied ratings, consistently find significant variation between a mattress's advertised firmness number and its measured compression relative to competitors' products at the same advertised number.

This is not evidence of dishonesty on any single manufacturer's part — it is a direct consequence of firmness rating not being a regulated, standardized measurement in the first place.

Some independent testers instead report a raw compression percentage alongside, or in place of, the manufacturer's 1-to-10 number specifically because the compression figure is measured the same way across every product tested, making it comparable across brands in a way the marketing scale is not.

These same testing organizations also typically retest a mattress after a period of simulated use, using mechanical rollers to approximate months or years of nightly pressure, which produces a second data point on how much a given model's measured firmness shifts as its materials compress over time.

A mattress firmness number is a real measurement, but one defined internally by whichever company assigned it, which is why it travels poorly between brands. Treating it as a rough starting point for comparison within one manufacturer's lineup, rather than a precise, universal specification, matches what the underlying test actually measures.

Note: This explains how sleep works as a system. It is not medical advice, it is not a diagnosis, and it is not a substitute for a licensed healthcare provider. Check the cited sources for current clinical guidance.