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How to Choose a Mattress Firmness
Side sleepers: 4–6/10 with cushioning at the shoulder. Back: 6–7. Stomach: 7–8. Weight and mattress depth (20–30 cm) change those numbers.
Updated 27 Aug 2026 · 6 min read · how to choose a mattress firmness
Position first, then body weight
Firmness is how much you sink before the support core catches you. Side sleeping needs a crater at the shoulder and hip so the spine stays a straight line from the side view. Back sleeping needs the lumbar filled without the hips dropping. Stomach sleeping needs almost no crater.
A 110 kg side sleeper on a 5/10 mattress may still bottom out; they want 6–7 with a pressure-relief comfort layer, not a 3/10 guest mattress. A 55 kg back sleeper on an 8/10 feels every coil. The shop’s “medium-firm” is often 6.5 — useful, not universal.
| Sleeper | Firmness band | Pillow loft to match |
|---|---|---|
| Side, average weight | 4–6 / 10 | 12–14 cm |
| Back, average weight | 6–7 / 10 | 10–12 cm |
| Stomach | 7–8 / 10 | 7–10 cm or less |
| Combination side/back | 5.5–6.5 / 10 | 11–12 cm adjustable |
| Heavier (+90 kg) | Add ~1 point | May need +1 cm loft on the side |
Topper versus replacement
A 5–8 cm latex or foam topper can drop perceived firmness by about one point. It cannot add support to a dead 8-year sprung mattress — that is when to replace a mattress. A protector is not a firmness tool; that split is protector vs topper.
Try it like a night, not a showroom sit
Lie in your actual position for 12–15 minutes. Sit-tests lie. If you buy online, use the 100-night trial and put the old mattress on the floor for a week so you can compare, not “decide in 40 seconds on day two.” Depth: a 25 cm mattress needs a fitted sheet with 35–40 cm pockets. Measure before you fall in love with percale that only stretches to 30 cm.
Questions, answered
- Is firmer better for a bad back?
- Not automatically. Many backs want medium-firm with lumbar fill, not a 9/10 board that leaves a gap under the small of the back.
- Do couples have to split the difference?
- Or buy a dual-firmness / zipper hybrid, or put a topper on one side. A 4/10 and an 8/10 cannot share one compromise foam without someone losing.
- Does a 30 cm mattress sleep firmer?
- Depth is not firmness. A 30 cm plush pillow-top can be a 4/10. Read the comfort spec, then the height for sheets.
