Medium density foam has a compression rate of 33lbs. 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. Medium firm quality foam is designed for heavy traffic and high use applications. 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 35 lbs. To pack it down 1" and have it return to its full 4" thickness has a 35 lb. Compression rating or ILD -indention load deflection. All foam sheets are sold per sheet - not per yard and rolled for packaging. Foam packages need to be opened immediately once you receive the package. Do not let the foam sit unopened for more than 1 week! 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. Foam can be cut to the size that you need with a sharp carving knife or electric knife. Will last 7 to 10 years. When unpacking, allow at least 2 days for the foam to breathe in order to recover its original size and form.