ILD is a measurement of firmness or compression. It is a measurement of how much weight it takes to compress foam. Technically it is how much weight it takes to depress a 50 square inch round metal disk 1” or 25% into a 4” x 16” x 16” piece of foam. It means indention load deflection; it is also known as indention force deflection (IFD). This standard was established by The American Society of Testing and Materials (ASTM) in order to establish a means for measuring firmness or compression. Is it a standard that tells you all you need to know about the firmness of foam? Not necessarily, it does not really tell you anything about support. In order to measure support, the disk is taken down to the 65% level. That measure of force or weight is divided by the 25% number to establish a measure called support factor. If the 65% number divided by the 65% number is 3 or greater, the foam is said to have excellent support. Some call this measure sag factor. You can literally have two piece of foam that have the same ILD rating buy vastly different support factor numbers. So ILD in and of itself tells you only one important measurement.
What does this mean to you, the consumer? Not a whole lot if that is the only thing the retail sales associate is telling you. Basically, ILD is a very small portion of the information you need when choosing a mattress. I really depends on where the foam is being used, as a mattress designer and manufacturer may help me in choosing where I am going to place the foam in the mattress. For softer feeling mattresses whether they are all foam, innerspring or hybrid, the measurement of ILD may give me a good indication if it will work well as a comfort layer. Generally, I prefer foams with softer ILD with lower support factor on the surface or comfort layers. For middle or transitional layers, I prefer more support factor regardless of which ILD I choose to use. For support or base layers, I generally want higher ILDs than comfort or transitional layers but am mostly concerned with foam that have high support factor.
In conclusion, don’t get so caught up in the technical details. I would be more concerned with how the bed feels. Is it soft and conforming on the surface with varying degrees of support once you lay into the mattress? Do you feel yourself hitting hard spots once you compress the surface layers of the mattress? Do you feel the transitional layer starting to support you without bottoming out in the support layers of the mattress? It is what many of us in the industry call “ride”, the feeling of pressure relieving surface comfort with varying degrees of support as you ride in the mattress.