Measured: Firmness has no standardized scale. It is described in words such as plush and firm, or on a seller's own 1 to 10 number. Both only rank beds within one brand's own lineup.
The basics
What to know about mattress firmness
Firmness describes how far the sleeping surface sinks under your weight, and it is the one thing about a mattress you cannot correct after delivery. A bed that is too firm accepts a topper. A bed that is too soft accepts nothing, because no pad will stop your hips dropping below your shoulders.
No industry standard defines these words. Plush, medium, cushion firm and firm are marketing labels, and one brand's firm lands where another's medium does. A published 1 to 10 number is that seller's own scale, not a shared one. Read the labels as an order within a single lineup, never across brands, and buy where there is a trial of at least 90 nights.
Judge by position and weight instead. Side sleepers load the shoulder and hip and need the surface to give there, so they trend softer. Back and stomach sleepers need the midsection held level and trend firmer. Weight moves the whole scale. A 130-pound sleeper sinks less and reads every bed as firmer. By contrast, a 250-pound sleeper compresses the same bed further and should shop one step up. The test takes ten minutes. Lie in your usual position and check that your spine holds the same line it holds when you stand.
