Why Do Mattresses With the Same Firmness Rating Feel Different?

Two mattresses can both be labelled "medium-firm" and feel nothing alike. One may let your shoulder settle into a soft top layer before a firmer core engages. Another may resist immediately at the surface. A third may feel comfortable to one sleeper and painfully hard to someone with a different body shape or sleep position.

That is why a mattress firmness scale is useful only as a starting point. There is no shared consumer scale that makes every brand's 5, 6 or 7 directly comparable. The number describes a brand's interpretation of the finished mattress; it does not tell you exactly how the layers will respond under your body.

The short answer

Mattresses with the same firmness rating feel different because the rating compresses several separate sensations into one label. Surface softness, sink depth, pressure relief, support-core resistance, bounce and ease of movement can all differ even when two products are called medium-firm. Your body weight, shape, sleeping position, bed base and time on the mattress change the experience again.

When comparing brands, do not ask only, "What number is it?" Ask:

1. How does the surface feel in the first minute?
2. How far do the shoulder and hip settle after 10 to 15 minutes?
3. Does the waist or lower back feel supported in your normal position?
4. Can you turn over without fighting the mattress?
5. Is the rating based on a whole-mattress test, a material specification or the brand's own description?

Why the same firmness label causes so much confusion

The problem appears clearly in owner discussions. In one Reddit thread about mattress firmness, a side sleeper said a firm model caused shoulder-blade pain and numbness even with a topper. Other owners described the same range very differently: one found the medium side "still fairly firm", another said the soft side remained too firm, while another thought a medium-plush version was extremely soft.

The sharpest complaint was that what the company called "medium" did not match other brands. Another owner said a topper made the mattress more tolerable for their partner but made them too hot.

These comments are anecdotes, not a controlled product comparison. They cannot establish which description is objectively correct. They do show the buyer's real problem: a single firmness word does not capture the mattress build, the sleeper or the thermal and movement trade-offs of changing the surface.

What a mattress firmness scale can and cannot tell you

Most consumer firmness guides run from 1 for very soft to 10 for very firm. Terms such as plush, medium, medium-firm and extra-firm are then mapped somewhere along that line. The apparent precision is misleading when the testing method, load and thresholds are not published.

A firmness rating can help you:

• Compare models within one brand when the brand uses a consistent method.
• Exclude an obviously unsuitable end of a range.
• Describe a broad preference when speaking with a retailer.
• Decide which models to test first.

It cannot reliably tell you:

• That two brands' 6/10 mattresses will feel the same.
• How soft the quilting or first comfort layer feels.
• How deeply your body will sink after several minutes.
• Whether the mattress will relieve pressure at your shoulder and hip.
• Whether the deeper support feels stable for your body.
• How easy it will be to change position.

ASTM International's F1566 mattress test methods include ways to evaluate firmness in components and finished mattresses. ASTM says the resulting data are for comparative analysis and do not create pass-or-fail comfort categories. In other words, mattresses can be tested consistently, but that does not turn retail labels such as "medium-firm" into a universal comfort grade.

Seven reasons medium-firm varies between mattress brands

1. The cover and quilting change the first impression

Your first contact is with the cover, quilting and uppermost comfort material, not the support core. Thick fibres, foam quilting or a plush panel can give a soft initial feel over a relatively firm mattress. A taut, lightly padded cover can make another mattress feel firmer immediately.

That first impression matters, but it is not the complete response. Stay in one position long enough to notice whether the deeper layers begin to carry your weight differently.

2. Layer order changes how firmness develops

Imagine two mattresses made from materials with similar individual hardness. One places a softer, thicker layer above a progressive transition layer. The other uses a thinner comfort layer directly over a resistant core. They may receive the same overall rating while producing different amounts of sink and different sensations at the shoulder, waist and hip.

A 2023 study in the Journal of Tissue Viability tested different mattress bedding-layer structures with 10 healthy adults. The design with hardness increasing from the top towards the bottom produced different pressure measures and higher subjective comfort scores than other arrangements. The small, short laboratory test cannot predict a full night's comfort for every sleeper, but it supports the underlying point: layer sequence affects the finished feel.

3. Different materials respond differently

Latex, polyurethane foam, memory foam and coils do not compress or recover in the same way. Two surfaces can feel similarly firm under a simple push yet differ in contour, bounce, response time and pressure distribution when a person lies down.

A National University of Singapore study compared latex and polyurethane mattresses using 20 participants in three sleeping postures for six minutes each. In that specific test, the latex mattress reduced peak pressure at several body regions and produced more low-pressure areas. The study compared two particular mattresses; it does not prove that all latex is softer, better or more comfortable than all polyurethane foam. It does show why a material label and one firmness number cannot describe the whole body-surface interaction.

4. A component test is not the complete mattress

Foam suppliers often use indentation force deflection or indentation load deflection measurements. These describe the force needed to compress a foam sample by a specified amount under defined conditions. They can be useful when comparing like with like.

ILD or IFD is not a universal score for the assembled bed. Sample thickness, test method and material behaviour matter, while the cover, quilting, other layers, adhesives, zoning and base all influence what you feel. Foam density is also not a direct firmness score: density describes mass per unit volume and may inform material comparison, but a denser foam is not automatically firmer.

If a retailer gives you an ILD figure, ask which layer it describes and under which test. Do not assume it converts neatly to a whole-mattress 1-to-10 rating.

5. Body weight and shape change compression

A mattress does not have one fixed feel independent of the person lying on it. A heavier body generally applies more load and may engage deeper layers sooner. A lighter body may remain closer to the surface and experience the upper layer as more resistant. Shoulder width, hip shape and how weight is distributed also affect local pressure.

An ergonomics study of objective firmness and subjective perception compared four mattresses and found relationships among measured firmness, average pressure and perceived firmness. It also reported a weak relationship between higher body mass index and sensitivity to changes in objective firmness. This was a short showroom-style test involving mattresses selected for older users, so it should not be turned into a rule that assigns one firmness to every weight band. Its practical message is narrower: the sleeper is part of the measurement.

6. Sleeping position moves the load

Side sleeping concentrates more load around the shoulder and hip. Back sleeping spreads contact across a different set of regions and places different demands on the waist. Stomach sleeping changes the question again because the pelvis and torso can settle differently.

The National University of Singapore study measured different pressure profiles in back, side and front postures. Other laboratory work has also found that firmness changes contact area and pressure distribution. These studies explain why a side sleeper can reject a mattress that a back sleeper of similar weight enjoys, but they do not identify one universally correct firmness for each position.

Use position as a testing instruction, not a shopping shortcut. Lie as you actually sleep and notice the specific regions that carry pressure.

7. The base becomes part of the sleep system

The same mattress can feel different on a rigid platform, closely spaced flat slats, sprung slats or a weak foundation. A flexible base can add give. Wide gaps or poor centre support can create uneven loading that feels like a mattress problem.

Always test on the type of base you will use at home, or ask the manufacturer how the showroom base differs. Follow the product's foundation rules; they also commonly affect warranty eligibility. Lullaflex's published warranty terms, for example, specify a firm, rigid and structurally sound foundation, slat gaps of no more than 5 cm, and centre support for queen and king sizes.

Firmness is not the same as support or pressure relief

These three ideas are often mixed together:

A firm mattress can still offer poor support if it is stiff in the wrong regions or rests on an inadequate base. A softer comfort layer can relieve pressure while a deeper core supplies resistance. Conversely, deep sink is not automatically good pressure relief if it makes movement difficult or leaves another region unsupported.

Research should also temper the familiar "medium-firm is best" headline. Some clinical studies have found benefits from medium-firm surfaces for groups with low-back pain, but a brand's medium-firm label does not confirm that its mattress matches the tested surface. If back discomfort is persistent, severe or worsening, use qualified medical advice rather than treating a retail firmness number as a diagnosis.

How to compare mattress firmness across brands

Use the same process for each shortlisted mattress. This creates a more useful comparison than trying to translate one brand's number into another's.

Step 1: Ignore the label for the first test

Ask the salesperson not to lead with the firmness number. Lie down first and record your own impression: surface feel, sink, pressure, support and movement. Then reveal the brand's rating. This reduces the chance that the label tells you what you expect to feel.

Step 2: Test in your real sleeping position

Spend at least 10 to 15 minutes in the position you use most. For side sleeping, notice the shoulder and hip. For back sleeping, notice whether the waist feels supported without a hard pressure point. If you change positions at night, test the transition as well.

This duration is a practical showroom method, not a scientific guarantee. A brief trial cannot reproduce body temperature, fatigue or several full nights at home.

Step 3: Separate the sensations

Score each mattress independently:

Do not collapse the notes back into one number. The separate observations are the point.

Step 4: Ask what was actually measured

Useful questions include:

• Is the firmness rating based on the finished mattress or one component?
• Is there a published whole-mattress test method?
• Are ILD, IFD or density figures available, and which layer do they describe?
• Does the model use zoning or different coil gauges?
• Will the display mattress feel different from a newly delivered one?
• Which base was used for the rating and showroom setup?

A retailer may not publish every engineering detail. The goal is not to collect impressive numbers; it is to find out whether apparently precise claims are actually comparable.

Step 5: Test together if the bed is shared

Partners can apply different loads, prefer different surface feels and disturb one another when turning. Both people should test in their normal positions at the same time. Record pressure, support, movement and usable space separately.

If preferences are far apart, consider split or adjustable constructions rather than forcing both sleepers to accept the same label. The Lullaflex Trilogy latex mattress, for example, uses separate soft, medium and firm latex layers that can be rearranged, with His/Hers configurations available in selected sizes. That adjustability creates options; it does not guarantee that one arrangement will suit every sleeper.

Step 6: Read the home-trial conditions before buying

A showroom comparison reduces risk but cannot recreate a complete night. Before relying on a trial, check its minimum-use period, total duration, exchange or return conditions, collection charges, hygiene requirements, product exclusions and whether a topper must be tried first.

Also ask what happens if your problem is pressure relief rather than overall firmness. Adding a topper changes the surface construction and may affect sink, movement and heat, as the Reddit owners discovered.

When can a topper fix a mattress that feels too firm?

A topper may help when the deeper mattress feels stable but the surface creates too much pressure. It adds another comfort layer, so its material, thickness and firmness determine how much the feel changes.

It is less likely to solve:

• A sagging or uneven support system.
• A mattress that already allows excessive pelvic sink.
• A weak or unsuitable bed base.
• A major mismatch between the sleeper and the complete construction.
• Heat discomfort caused or worsened by adding more insulating material.

Treat a topper as a construction change, not a guaranteed softness switch. If a new mattress already needs several corrective purchases, review the return terms before spending more. Our mattress buying mistakes guide covers the wider checks that labels and discounts can distract from.

Frequently asked questions

Is mattress firmness standardised across brands?

No universal consumer rating makes one brand's medium-firm directly equal to another's. Standardised laboratory methods can measure aspects of components or finished mattresses, but brands may use different methods and thresholds when assigning retail terms or 1-to-10 scores. Compare the complete construction and your response, not the label alone.

What does medium-firm mean on a mattress firmness scale?

Medium-firm usually describes the firmer side of the middle range, often shown around 6 or 7 on a 10-point brand scale. That placement is not universal. It does not reveal surface softness, sink depth, pressure relief or support-core behaviour, so use it to shortlist mattresses rather than predict an exact feel.

Why does a mattress feel firmer to a lighter sleeper?

A lighter sleeper may compress the upper materials less and may not engage the deeper transition layers in the same way as a heavier sleeper. Body shape and position also change local pressure, so weight alone cannot prescribe firmness. Test the mattress in your normal position and focus on pressure and support.

How can I tell if a mattress is too firm for side sleeping?

Warning signs can include persistent pressure or discomfort at the shoulder and hip, difficulty settling into one position, or a visible gap where the waist does not feel supported. These signs are not a medical test. Persistent numbness, pain or worsening symptoms warrant advice from a qualified healthcare professional.

Does a higher foam density mean a firmer mattress?

Not necessarily. Density measures mass per unit volume, while firmness describes resistance under a defined compression test or a perceived whole-mattress feel. Two foams can have different density and firmness combinations. Ask which material a figure describes and avoid using density as a direct conversion to a retail firmness score.

Should I choose a mattress based on ILD?

ILD or IFD can help compare similar foam samples tested under the same method. It does not account for the complete mattress, including quilting, layer thickness, zoning, support core and base. Use it as one material specification, not as a substitute for testing the assembled bed.

Conclusion

Mattresses with the same firmness rating feel different because "firmness" is not one sensation and retail scales are not universally interchangeable. Materials, layer order, cover construction, sleeper characteristics, sleep position and the bed base all change the result.

Use the mattress firmness scale to create a shortlist. Then compare pressure relief, deeper support, movement, heat and trial terms separately. The best question is not whether two brands both say medium-firm. It is whether the complete sleep system works for the people who will use it.

Sources

• Reddit: Experiences with Ecosa beds and their firmness?: owner experiences; anecdotal evidence, not a controlled comparison.
• ASTM International F1566: Standard Test Methods for Evaluation of Innersprings, Boxsprings, Mattresses or Mattress Sets: recognised component and finished-mattress test methods; comparative data without comfort pass/fail criteria.
• López-Torres et al., Objective firmness, average pressure and subjective perception in mattresses for the elderly: short-term comparison of objective firmness, pressure and perceived firmness.
• Low et al., Effects of Mattress Material on Body Pressure Profiles in Different Sleeping Postures: 20-participant comparison of two mattress materials across three postures.
• Effect of mattress bedding layer structure on pressure relief performance and subjective lying comfort: small laboratory study comparing different layer structures.
• Lullaflex mattress warranty: current foundation and slat-support requirements.
• Lullaflex Trilogy natural latex mattress: current layer construction and adjustable options.