High density foam has a compression rate of 44lbs. It features the perfect amount of cushioning and stiffness. Its soft but still holds its firmness and support for an individual to sit on it without compressing the foam too much. Compression = firmness compression (not density) is what determines firmness. If you dont want a cushion thats hard as a rock or a mattress that sinks when you sit on it, you have to check the compression or ild number. Compression means the number of pounds necessary to compress a piece of foam 25% and still bounce back to the original thickness. For example, a 4" thick piece of foam that requires 33 lbs. To pack it down 1" and have it return to its full 4" thickness has a 33 lb. Compression rating or (ild) -indention load deflection. - all foam sheets are sold per sheet - not per yard. Do not open with a box cutter. Use a ball point pen or a pencil. Start by making a little incision and break it from there - durability : will last 7 to 10 years Medium density foam is used for firmness yet relax able cushion. It has high quality compression and material to keep it lasting for longer periods of time even when harsh weight is applied.