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How Is Mattress Firmness Measured?

Last updated Researched from published specifications and owner reviews, not tested in person

Quick answer

Mattress firmness is not measured, it is declared. No standards body defines a firmness test for a finished mattress. Each manufacturer places its own models on a 1 to 10 scale relative to the rest of its own lineup, so a "6" from one brand and a "6" from another are not the same feel.

This is the single most misunderstood number in the entire mattress category, and it is worth being blunt about it up front: the firmness rating on a listing is marketing copy the manufacturer wrote about its own product, not a lab result. That does not make it useless, but it does mean the number carries a very different kind of authority than a mattress's width or length.

On this page
  1. Is there an actual test for mattress firmness?
  2. What does ILD actually measure?
  3. The 1 to 10 firmness scale as manufacturers use it
  4. If firmness is not measured, what actually guides a choice?
  5. Mattresses across the declared scale

Is there an actual test for mattress firmness?

No. There is no ISO, ASTM, or any other independent body that tests a finished mattress and assigns it a firmness number. What exists instead is a self-reported scale: a manufacturer builds a lineup of models at different feels, then ranks them from softest to firmest on a 1 to 10 scale relative to each other. That ranking is genuinely useful within one brand's own catalog, because it tells you which of their models is softer or firmer than which. It tells you nothing reliable about how their "7" compares to a different brand's "7".

What does ILD actually measure?

ILD, Indentation Load Deflection, is a real laboratory measurement: it is the force needed to compress a foam sample by 25 percent of its thickness. It is a genuine number with a defined test method, which is why it sometimes gets cited as if it settles the firmness question. It does not, for two reasons. First, ILD applies to a single foam layer, not a finished multi-layer mattress, so it describes one ingredient rather than the assembled product you sleep on. Second, most manufacturers simply do not publish it at all, so even where it exists it is rarely something you can shop by.

The 1 to 10 firmness scale as manufacturers use it

The firmness scale as it is commonly used
NumberLabelWho it usually suits
2Very softRarely sold. Sinks deeply.
3SoftLighter side sleepers.
4Medium softSide sleepers who want pressure relief.
5MediumCombination sleepers.
6Medium firmThe most widely sold feel. Back and combination sleepers.
7FirmBack and stomach sleepers, heavier sleepers.
8Very firmStomach sleepers and those who want minimal sink.

Convention, not a standard

This is widely repeated guidance rather than a measured standard. Treat it as a starting point.

This is the firmness scale as it is commonly repeated across the industry. It is a shared convention for talking about feel, not a measurement any two brands calibrate against each other.

If firmness is not measured, what actually guides a choice?

Sleep position and body weight are the two factors with the most repeated, if still conventional, guidance behind them. Heavier sleepers compress a given surface more, so they generally want a firmer rating than a lighter sleeper to land on the same effective feel.

Commonly suggested firmness ranges by position and weight
PositionUnder 130 lb130 to 230 lbOver 230 lb
Side3 to 54 to 65 to 7
Back5 to 66 to 76 to 8
Stomach6 to 76 to 87 to 8
Combination4 to 65 to 76 to 8

Convention, not a standard

This is widely repeated guidance rather than a measured standard. Treat it as a starting point.

This is guidance repeated across the industry, not a measurement, and it assumes the ratings being compared are from the same brand. Use it as a starting range, then rely on a trial period to confirm the actual feel with Mattress Firmness Calculator.

Mattresses across the declared scale

Frequently asked questions

How is mattress firmness measured?

It is not measured by any independent test. Each manufacturer assigns its own models a number on a 1 to 10 scale relative to the rest of its own lineup, which makes the number a description rather than a measurement. There is no standards body that certifies mattress firmness the way there is for, say, mattress size, so treat the rating as a starting point rather than a specification.

What does ILD mean on a mattress listing?

ILD, Indentation Load Deflection, is a genuine lab measurement of how much force compresses a foam sample by a set percentage. It is real, but it measures a single foam layer, not the finished mattress you would sleep on, and most brands never publish it. Where it does appear, treat it as one data point about one component rather than the mattress's overall feel.

Why does a "6" feel different between two brands?

Because each brand sets its own 1 to 10 scale against its own lineup rather than against any shared reference. A "6" only tells you that model sits in the middle of that particular brand's range, not what physical firmness it corresponds to. This is exactly why trial periods matter more in this category than the number printed on the box.

What firmness should I choose for my weight and position?

As a starting range: side sleepers generally want softer (3 to 7 depending on weight), back and combination sleepers land in the middle (4 to 8), and stomach sleepers and heavier sleepers generally want firmer (6 to 8). These ranges are industry convention rather than a measurement, so use them to narrow a shortlist and confirm the actual feel during a trial period.

Can firmness change over time?

Yes. Foam softens as it breaks in over the first few weeks, and most materials continue to soften gradually over years of use as cell structure compresses permanently. A mattress that felt firm out of the box commonly feels a point or so softer after a couple of months, which is a separate effect from the manufacturer's stated rating and is expected rather than a defect.

Is medium firm really the most popular?

It is the feel most widely stocked and marketed, generally sitting around a 6 on the common scale, because it suits the largest overlapping group of back and combination sleepers. Popularity is not the same as correctness for any individual: side sleepers and heavier or lighter sleepers commonly do better outside that middle band, based on position and weight rather than what sells the most.

How to read the numbers on this page. Standard mattress and bedding dimensions are industry-wide and the same across manufacturers. Guidance like replacement intervals, firmness by body weight and walking clearance is convention rather than a measured standard, and every table here says which it is. Where a manufacturer does not publish a figure we leave it out instead of estimating. This is not medical advice: a mattress is not a treatment for a medical condition, and persistent pain is a reason to see a clinician.