All foam is defined by a series of numbers that reflect the weight and compression ratios of a square foot of foam. Most common in the industry for testing purposes, is a piece of foam 4x12x12square. Precision equipment is used in determining these ratios, the 1st two numbers represent density and this is the weight in pounds per cubic foot. The 2nd two numbers represent the compression rate. If the second number is 26 then it takes 26 lbs. To compress it from 4 to 3 or 25%, this is commonly known as the IFD, or Indentation Force Deflection rating. This translates, the higher the last two numbers (or the IFD rating) the firmer the foam will be. For example our 2550 foam type has a 2.5 density and a 50lb. compression rating. At www.foamproducts.net, all of our foam are quality foams that we have been selling to the trade for over 25 years, so you can be assured when purchasing you are getting quality and affordability.