Definition of IDF (Indention Force Deflection)

Meaning of the Term as it Pertains to Mattresses

What is Indention Force Deflection?

Indention Force Deflection, or IFD, is a measurable representation of how firm or soft the surface of a piece of foam feels. Also known as ILD (initial load deflection), IFD expresses comfort level as a number. Memory foam mattress IFD numbers usually range from 8 to 16. The higher the number, the firmer the foam feels.

IFD is evaluated by measuring how much weight is necessary to make a piece of foam compressed by 25%. A test is done by placing a weighted circular disk at the center of a piece of foam and measuring the foam's height. Weight is added until the foam has compressed by 25% (or the foam has reduced to 75% of its original height). The amount of weight it takes to reach 25% compression is the ILD number, which means if it takes 12 lbs. to compress the foam to 25%, the mattress has 12 IFD.