Firm isn't better. Medium-firm is.

That's the advice most people have been given and it's out of date. Where trials have compared them, medium-firm generally performs better than firm for back pain, and the old suggestion of putting a board under the mattress isn't supported. It's also worth knowing the evidence in this area is weak, inconsistent and often industry-funded — which is exactly why the approach here is narrowing the field and then testing properly, rather than calculating an answer from your weight.

Not honestly, and here's the reason: there is no standardised firmness scale, so one brand's medium-firm is another's firm and the number on a website isn't comparable between them. There's also no validated method for prescribing a mattress from body weight and sleeping position — that's a sales methodology rather than a clinical one. What does predict outcome reasonably well is what you personally find comfortable, which is why testing beats specification.

Two tells. Pain that's clearly better after sleeping somewhere else and returns at home is the suggestive one. Pain that's worse on waking but eases within half an hour of getting up is common and frequently not the mattress at all. There's also a physical check: a dip of more than about two or three centimetres, measured with a straight edge rather than by eye, means it's failed structurally regardless of age.

The comparison ranks four approaches, and going straight to a new mattress comes last — not because mattresses don't matter, but because it's the most expensive and least reversible way to test an unchecked hypothesis. In cost order: the pillow, the base, then a topper. An old or sagging base in particular will reproduce the same problem in a new mattress within weeks, and can void the warranty.

That it's long enough to be real. Bodies take a few weeks to adapt to a different surface and the first nights are often worse, so a trial under about a month tests nothing — the calculator shows how many genuine testing nights you have left once adaptation is subtracted. Then the small print: who pays return shipping, whether collection is included, restocking fees, whether a protector is required for the trial to be valid, and whether it's a refund or credit.