ILD (Indentation Load Deflection)
The unit of measurement refers to the number of pounds (lbs) of pressure that is needed to indent foam to a depth of 4” by 25%. This test is performed on an area of 50 square inches. The rating is reffered to as the ILD rating or the 25% ILD rating.
Most people sleep comfortably on mattresses that have a 25-35 ILD.