How to Firm Up a Soft Mattress (And Why It Usually Fails)

Here is the uncomfortable answer first: you cannot really firm up a soft mattress. You can firm up the surface, which is a different thing, and if the problem is the support core then nothing on this page will fix it.

That asymmetry catches people out. A topper genuinely rescues a too-firm mattress, because you are adding give on top of support. Adding a firm layer on top of a soft mattress gives you a hard crust over a soft middle — which most people find worse than what they started with.

First: is it soft, or is it sagging?

These need different answers and people confuse them constantly.

• Soft means the whole surface gives more than you want, evenly.
• Sagging means there is a hollow where you lie and a ridge where you do not.

Lay a broom handle across the mattress. Daylight underneath in the middle means sagging, and a sagging mattress is finished — see when to replace it. Everything below assumes it is genuinely just soft.

What actually helps, in order

1. Check the base first

The single most common cause of a bed that has gone soft is not the mattress. Take it off and look at what is underneath. A dropped or snapped slat, a sagging divan platform, or slats more than 7 cm apart will all make a firm mattress feel soft. This is the cheapest fix on the page and the most likely to work.

2. Move it to the floor for a week

Not a permanent solution — no airflow underneath — but it tells you definitively whether the base or the mattress is the problem. If it feels right on the floor, buy a base.

3. Firm latex topper

The only topper material with a real chance, because latex pushes back rather than compressing. Four centimetres of firm latex over a moderately soft mattress does make a difference. Over a very soft one it does not — it just sinks with it.

4. Turn the heating down

If the mattress has a memory foam comfort layer, it is temperature-sensitive by design. A bedroom at 22 °C makes it markedly softer than one at 17 °C. Worth trying before you spend anything.

5. Rotate it

Head to foot. Moves you onto the less-compressed half. Buys months, not years.

What does not work

• Plywood under the mattress. The classic advice, and it does firm things up — by removing all airflow underneath. Expect condensation and, within a season or two, mould on the underside. Use it as a diagnostic for a week, not as a fix.
• Putting the mattress on the floor permanently. Same problem.
• A thick fibre topper. Adds softness, which is the opposite of what you asked for.
• Letting it "settle". Mattresses get softer with age, never firmer.

If it is a new mattress and it is too soft

Do not start modifying it. Use the trial. A mattress that is wrong on firmness in the first month will still be wrong in the sixth, and every fix above is a compromise compared with getting the right one. Our trial is 100 nights with free collection, and the honest use of it is exactly this.

Common questions

Will a mattress firm up over time?

No. Foams soften and springs lose tension. It only goes one way.

Is a bed board the same as plywood?

A proper bed board is slatted or perforated, which is better than solid ply, but it still reduces airflow.

What firmness should I have bought?

See firm vs medium vs soft — position and weight decide it.