The Mattress Firmness Scale Explained

No. There is no industry-wide test or certification behind the 1 to 10 firmness number on a mattress listing. Each manufacturer sets its own scale relative to the softest and firmest mattresses it sells, so a 6 from one brand is not guaranteed to feel the same as a 6 from another. Treat the number as useful only for comparing mattresses within the same brand's own lineup.

ILD stands for indentation load deflection, a lab measurement of how much a specific piece of foam compresses under a set load. It is a real, physical number, but it describes one foam layer tested by itself, not a finished mattress built from multiple layers and a cover. Most mattress brands do not publish an ILD figure at all, and it cannot be compared directly across brands even when they do.

Widely repeated guidance puts side sleepers in the 3 to 7 range depending on body weight: lighter side sleepers, under about 130 lb, often prefer 3 to 5, while heavier side sleepers, over about 230 lb, often need 5 to 7 for enough support. This is convention rather than a measurement, and since firmness numbers vary by brand, the actual feel should still be confirmed during a mattress's trial period.

Medium firm, generally described as a 6 on the 1 to 10 scale, is the most widely sold feel across the mattress industry. It tends to suit back and combination sleepers and is the default many brands build their flagship model around. That said, the number is set independently by each brand, so a "6" is a starting point for what to try, not a guarantee of exactly how a specific mattress will feel.

You can trust it as accurate within that brand's own lineup, since a company's 8 will reliably feel firmer than its own 4. What you cannot do is compare that number directly to a different brand's scale, because there is no standardized test setting either one. Use the number as a rough starting point, then rely on the mattress's trial period to confirm the actual feel matches what you expected.

Yes, somewhat. New foam mattresses commonly need roughly 24 to 72 hours just to fully expand after being compressed for shipping, and manufacturers often describe an additional adjustment period, commonly up to about 30 nights, before the foam softens slightly and settles into its intended feel. A mattress that feels firmer than expected on the first night is not necessarily the wrong firmness; give it the full break-in window before judging.