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.
The higher the IFD or ILD is, the firmer the foam is. For example, the foam for a bed’s core usually has an IFD of 30 or higher. A car seat may have a IFD of 40 – 50 or even higher. Memory foam, which is obviously softer than a car seat, is usually found with an IFD of 10 – 15. Density and IFD are not related and do not have to be the same for foams with the same densities. Foams can also have the same density and different IFDs. For example, our first two layers of memory both have IFDs of 13, but the density of the top layer is 4-lb and the second layer is a 5-lb density. Same IFD, but different densities. Both of our base layers are 2.3-lb densities, but the bottom layer has an IFD of 50 (firmest layer) and the layer just below the memory foam has an IFD of 31. Same density, but different IFD for the base layers with the bottom layer being the firmest.
Foams with lower IFDs will be softer than foam with higher IFDs. If you are looking for a softer mattress, look for base foams with a 21 or 26 IFD. If you want firmer base foams, look for higher IFDs.
To learn more about memory foam, checking out our Memory Foam Buying Guide.