ILD stands for (Initial Load Deflection). It is the measure of firmness (softness and hardness) of a foam mattress. To determine the ILD, a foam block 15 inches square and 4 inches high is pressed down 1 inch by an 8 inch disc. The pressure reading for pressing the foam down 1 inch is the ILD. The higher the ILD, the firmer the mattress. In general terms, mattress firmness can be measured approximately thus: Soft – 16 ILD, Medium – 26 ILD, Medium Firm – 35 ILD, Firm – 41 ILD, Extra Firm – 61 ILD.