How To Choose A Mattress Firmness That Actually Suits You

What Firmness Ratings Mean

Most UK mattress retailers rate firmness on a scale from soft to extra firm, usually with four or five steps. There is no industry standard, so a "medium" from one brand might feel quite different from a "medium" from another.

The broad categories are:

• Soft (1 to 3 out of 10): deep sinkage, strong contouring, very little pushback
• Medium (4 to 5 out of 10): some sinkage with moderate support underneath
• Medium firm (6 to 7 out of 10): light sinkage, noticeable support, the most popular rating in the UK
• Firm (8 to 9 out of 10): minimal sinkage, strong pushback
• Extra firm (10 out of 10): almost no give, often recommended for specific medical conditions

Medium firm accounts for roughly half of all mattresses sold in the UK. It works well for a wide range of body types and sleeping positions, which is why retailers default to it.

Firmness By Sleeping Position

Your sleeping position is the single biggest factor in choosing firmness.

• Side sleepers need a softer mattress (soft to medium). Your shoulders and hips carry most of your weight when you lie on your side, and they need to sink into the mattress enough to keep your spine aligned. A mattress that is too firm will create pressure points at the shoulder and hip.
• Back sleepers suit medium to medium firm. Your weight is more evenly spread, so you need less contouring but enough give to support the natural curve of your lower back.
• Front sleepers need a firmer mattress (medium firm to firm). A soft mattress lets your hips drop below your shoulders, which puts strain on your lower back. A firmer surface keeps your spine closer to a neutral line.
• Combination sleepers who change position through the night generally do best with medium or medium firm, since it works across multiple positions without being wrong for any of them.

If you are unsure which construction type suits your sleeping position, our memory foam vs pocket sprung guide explains how each mattress type feels in different positions.

Body Weight And Firmness

Weight affects how deep you sink into a mattress, which changes how firm it feels. The same mattress will feel softer to a heavier person and firmer to a lighter one.

As a rough guide:

• Under 60 kg: consider one firmness level softer than the general recommendation for your sleeping position
• 60 to 100 kg: the standard recommendations above should work well
• Over 100 kg: consider one firmness level firmer, and look for mattresses with higher-density foam or higher spring counts for durability

Couples with a significant weight difference sometimes struggle to find a single mattress that suits both. A zip-and-link design (two single mattresses joined inside a king-size frame) lets each partner choose a different firmness.

Back Pain And Firmness

If you have back pain, the instinct is often to choose a firm mattress. Research suggests this is usually wrong. A 2003 study published in The Lancet found that people with chronic lower-back pain reported better outcomes on medium-firm mattresses than on firm ones.

The key is spinal alignment, not hardness. Your mattress should keep your spine in its natural S-curve whether you sleep on your back or your side. A mattress that is too soft lets your spine sag; one that is too firm forces it into an unnatural position.

If you wake up with stiffness that eases within thirty minutes of getting up, your mattress firmness is likely the cause. If pain persists throughout the day, speak to a physiotherapist before blaming the mattress.

How To Test Firmness

The best way to judge firmness is to lie on the mattress for at least ten minutes in your normal sleeping position. Showroom tests are useful but limited, since you are fully clothed, alert, and surrounded by shop noise. Your body behaves differently when relaxed and asleep.

Many UK mattress brands now offer trial periods of 100 to 200 nights. These are genuinely useful for testing firmness, especially if you are switching to a type you have not tried before. Check the return conditions carefully. Some brands charge a collection fee, and a few require you to keep the mattress for a minimum period (usually 30 nights) before returning it.

When testing in a shop:

• Wear comfortable clothing and remove your shoes
• Lie in your usual sleeping position, not just on your back
• Spend at least five minutes on each mattress you are considering
• Bring your partner if you share a bed, as their weight changes how the mattress feels

Common Mistakes

• Choosing "firm" because it sounds supportive. Firmness and support are different things. A good mattress supports your spine at any firmness level; the rating describes feel, not quality.
• Judging firmness by pressing with your hand. Your hand has very different weight distribution from your torso. Always lie down.
• Assuming your old mattress was the right firmness. If you slept poorly on it, the replacement should probably be different.
• Ignoring the comfort layers. The top 5 to 8 cm of a mattress determines how it feels on first contact. Two mattresses with identical bases can feel completely different depending on what sits on top.

If you are ready to start comparing, our mattress listings page lets you filter by firmness across UK retailers. For help choosing the right size, see our UK mattress sizes guide.