ILD (Indentation Load Deflection)

ILD (Indentation Load Deflection) is the standardized firmness measurement for foam — used across memory foam, polyfoam, and latex industries. The test compresses a 4" thick, 50 sq in foam sample by 25% of its height; the force required (in pounds) is the ILD rating.

Typical ranges: comfort layer foams run 8-16 ILD (very soft to medium-soft), transition foams run 18-26 ILD (medium-firm), and support core foams run 28-44 ILD (firm to very firm). Latex spans the entire range from 14 ILD to 44 ILD.

ILD is not the same as 'mattress firmness'. A mattress's overall feel depends on the ILD of every layer, the thickness of each layer, and the order they are stacked. A 14 ILD comfort layer over a 36 ILD support core feels softer than a 14 ILD layer over an 18 ILD core.

Comparing ILD across foam types requires care. A 28 ILD memory foam feels softer than a 28 ILD latex due to memory foam's slow recovery — the pressure spec is the same but the perceived feel differs.

ILD is measured by pressing a 50 in² disc four inches into a foam sample and recording the force in pounds — a lab number rather than a sleeping experience. It describes one layer in isolation, which is why an 'ILD 12' comfort layer over a firm core can still produce a medium-firm mattress overall.

Use ILD when configuring modular or split-firmness beds, where you are literally choosing layers, and largely ignore it when comparing finished mattresses across brands. Comparative firmness on the 1-10 scale, measured on the assembled bed, is the more honest number for shopping.