ILD ratings are usually expressed in ranges between 10 to 16: 10 to 12 (soft) 13 to 15 (medium) 16 and above (firm)
Understanding the Rating
For the 10 to 12 range, the ILD reading indicates that it takes 10 to 12 lbs. to indent the foam by 25 percent. A higher ILD rating does not signify a better mattress; it only says the mattress is firmer.
How to Use the ILD Rating
Use it as a guide. If you know you prefer a medium mattress, start looking at mattresses with ILD ratings of 13 to 15, but don't limit yourself to these ratings. Mattress ratings and compositions vary.
ILD and Density
An ILD rating does not reflect mattress density. The density of a mattress relates to foam composition and determines how quickly the foam recovers or springs back into shape.
When choosing a mattress, remember that the ILD rating is only one consideration. You will want to take density, price and comfort into account as well.