Foam is identified by a 4-digit number. The first 2-numbers tell you the foam’s density, like: 1.0 pound/cubic foot, 1.5 pounds, 1.8 pounds, and so on.
The Density refers to the weight of the foam and not its firmness. The last 2 numbers tell you the compression or ILD of the foam. The higher the last 2 numbers the firmer the foam will feel. The firmness that you choose also depends on your required thickness. Watch to learn more.