IFD (Indentation Force Deflection), also known as ILD, is the measure of the firmness of a foam. It is used in everything from memory foam to car seats. IFD/ILD measure the deflection of a foam under a load. The specific IFD test gives the foam a numeric IFD value. The higher the number, the firmer the foam. This means, for example, that memory foam will have a much lower number than a base foam for a mattress and a car seat will have a higher number than mattress base foam.
When you hear about foams, particularly memory foam, you usually always hear about their densities, but density isn’t the only measurement used in assessing foams. IFD, also called ILD, is the measure of a firmness of a foam. It measures the initial deflection of the foam under a load and is used for all types of foam, everything from memory foam to car seats.