Mattress Firmness Test: Measured Indentation Data vs Marketing Labels

Mattress firmness labels are descriptions, not measurements. Soft, medium and firm are chosen by the manufacturer to describe a product within its own range, and nothing obliges two companies to place the boundaries in the same place. A medium from one brand can be closer to another brand’s firm than to its medium.

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There is a measured property underneath the marketing, and it is worth understanding. Indentation testing compresses a foam sample by a set proportion of its thickness and records the force required. It produces a repeatable number, but that number describes one layer of foam, not a finished mattress. We did not conduct a laboratory testing programme for this report, and we publish no test figures here. What we can set out is what the measurement means, what it leaves out, and how to judge a mattress without relying on the label.

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Key findings at a glance

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• Firmness labels are not standardised between manufacturers and cannot be compared directly across brands.

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• Indentation testing measures the force needed to compress a foam sample by a defined percentage of its thickness, which is a property of that foam alone.

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• A finished mattress combines several layers plus a cover, so no single indentation figure describes the whole product.

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• Firmness and support are different properties: firmness is how the surface feels, support is whether the spine stays aligned.

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• Body weight changes perceived firmness substantially, which is why two people can describe the same mattress in opposite terms.

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• Sleeping position is the more reliable starting point for selection than any label on the product.

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• UK mattress sizes are standardised, so size selection is straightforward even where firmness is not.

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What indentation testing actually measures

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The standard laboratory approach compresses a flat foam sample with a circular indentor and records the load at a set compression, typically expressed as the force needed to reach a given percentage of the sample thickness. Repeat the test on the same foam and you get the same result, which is what makes it useful to manufacturers specifying materials.

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The limits are equally clear. The test uses a flat sample of one material at a controlled thickness, and it applies a rigid circular indentor rather than a human body. A person lying on a mattress applies an irregular, unevenly distributed load across a large area, and the layers beneath the surface respond in sequence: the cover and quilting first, then the comfort layer, then the support core. The feel that results is the sum of all of those, and no single layer figure predicts it.

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A second limitation is that the relationship between load and compression is not linear in most foams. A material that feels soft under light pressure can become firm quickly as it compresses, which is precisely the behaviour good comfort layers are designed to have. Two foams with a similar test figure at one compression level can feel quite different when a person lies on them.

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Firmness is not support

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The confusion that causes most poor purchases is between firmness and support. Firmness is a surface property: how much the top of the mattress yields when pressed. Support is a structural property: whether the mattress holds the spine in the alignment it has when standing.

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The two can move in opposite directions. A soft surface over a well specified support core can hold a sleeper in good alignment while feeling yielding. A hard surface over a weak core can feel firm and still allow the hips to sink. This is why the common advice that a bad back requires a firm mattress is unreliable. What a back usually requires is a support core that resists the heaviest part of the body while a comfort layer allows the shoulder and hip to sit into the surface.

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It also explains why the base matters. A mattress on a solid platform behaves differently from the same mattress on a sprung slatted base, and a slatted base with wide gaps changes the support pattern again. Where a mattress is being replaced and the problem persists, the base is worth examining before the mattress is blamed. Any of our modern beds UK will state the slat type and spacing, and that is the figure to check.

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Why the same mattress feels different to different people

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Perceived firmness is the result of pressure, and pressure is weight divided by contact area. A heavier person applies more force over roughly similar contact area, compresses the comfort layer further and reaches the support core sooner, so the mattress feels firmer and less cushioning. A lighter person may not compress the comfort layer enough to engage the support core at all, so the same mattress feels harder than intended, because they are effectively lying on the surface rather than in it.

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Sleeping position compounds this. A side sleeper has a narrow contact area at the shoulder and hip, and needs the surface to yield enough at those points to keep the spine straight across the body. A back sleeper spreads load over a much larger area and needs the lumbar curve supported rather than filled. A front sleeper needs the hips prevented from sinking, which means a firmer surface than either of the others.

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For couples with a significant weight difference, no single conventional mattress satisfies both fully. Options are a zoned or dual tension product, two single mattresses on a linked base, or accepting a compromise weighted towards the heavier sleeper, who will notice an unsuitable core sooner than the lighter sleeper will notice an unsuitable surface.

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Starting points by sleeping position and weight

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The table below gives a sensible starting point rather than a prescription, matching sleeping position and build to the surface behaviour and support requirement that usually suits. It is based on body mechanics and standard mattress construction, not on test scores.

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Sleeping position	Build	Surface behaviour wanted	Support core requirement	What to check when lying down
Side	Lighter	Yielding at shoulder and hip	Moderate	Spine straight when viewed from behind
Side	Heavier	Yielding but deep enough not to bottom out	Firm, zoned if available	Hip not dropping below shoulder line
Back	Lighter	Light contour, lumbar contact maintained	Moderate	Hand slides under the lower back with slight resistance
Back	Heavier	Minimal surface sink	Firm	Hips level with the shoulders and chest
Front	Any	Firm surface, little contour	Firm	Lower back not arching
Mixed position	Any	Moderate contour with quick recovery	Moderate to firm	Ease of turning without pushing out of a hollow

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What this means for UK homes

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Test by alignment rather than by first impression. Lie in your usual sleeping position for several minutes, not seconds, and have someone look along the line of the spine from behind. A straight line from the neck through to the base of the spine is what a correctly matched mattress produces, and it is a far better test than whether the surface feels pleasant to the hand.

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Ignore cross brand label comparisons. Compare within a single range, where the manufacturer’s soft, medium and firm do at least relate to each other consistently, and treat descriptions from different makers as unrelated.

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Check the base at the same time. Slat spacing, base type and the age of the existing frame all change how a mattress performs, and many manufacturers set out a maximum slat gap for their warranty to apply. Replacing a mattress onto a tired base is the most common way a good mattress underperforms.

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Buy to UK standard sizes, which are consistent and make bedding straightforward: single at 90 x 190 cm, small double at 120 x 190 cm, double at 135 x 190 cm, king at 150 x 200 cm and super king at 180 x 200 cm. Check the room can take the size chosen with a walkway of at least 60 cm (24 in) on any side used for access, and confirm that the mattress and the bed frame can both be carried up the stairs. Our modern mattresses UK listings give depth as well as length and width, which matters when fitted sheets and storage under a modern ottomans UK base are involved.

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Methodology

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This report was prepared in 2026 by the editorial team at Furniture in Fashion. Despite the testing framing of the title, we did not carry out a laboratory measurement programme, and no indentation figures, firmness scores, brand comparisons or test results are published here, because we have no verified measurement data of our own to report and we do not estimate figures. The analysis explains the principle of indentation testing as used in foam specification, sets out the mechanical distinction between firmness and support, and gives standard UK mattress dimensions. It also reflects our own editorial assessment as a UK furniture retailer. No proprietary sales data, customer data or survey fieldwork is used.

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Citing this report

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Journalists and websites may cite this analysis with credit to Furniture in Fashion and a link to the report. Please describe it as an explanation of firmness measurement and labelling rather than as a product test, since we publish no measured results. Further comment on mattress selection by sleeping position is available on request.

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Frequently asked questions

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Is there a standard firmness scale for mattresses?

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Not one that applies across manufacturers. Firmness descriptions are set by each maker for their own range, so a medium from one brand and a medium from another are not comparable. Within a single range the descriptions are at least internally consistent.

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Does a firm mattress help back pain?

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Not necessarily. What matters is spinal alignment, which comes from the support core rather than from surface hardness. A soft surface over a strong core can hold alignment better than a hard surface over a weak one. Persistent pain is a matter for a clinician rather than a retailer.

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Why does my new mattress feel firmer than the one in the showroom?

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Showroom samples have usually been compressed by repeated use, which softens the comfort layer. New foam and new springs also need a settling period, and most manufacturers state a break in period during which the surface changes. Base type differences between showroom and home account for much of the rest.

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What mattress suits two people of very different weights?

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A zoned or dual tension mattress, or two single mattresses on a linked base, handle the difference better than a single conventional mattress. If one product must be chosen, weight the decision towards the heavier sleeper, who is more likely to be affected by an unsuitable support core.

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What are the standard UK mattress sizes?

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Single is 90 x 190 cm, small double 120 x 190 cm, double 135 x 190 cm, king 150 x 200 cm and super king 180 x 200 cm. These are consistent across UK retailers, which makes bedding and frame compatibility straightforward.