Look for the compression measurement or firmness of the polyurethane foam. Firmness is based on how much pressure is needed to compress the foam by 25 percent. The firmness level will be an indicator of the best use for the foam. The Polyurethane Foam Association (PFA) says this test is called the indentation load deflection (ILD) or indentation force deflection (IFD). Medium to extra firm is good for seat cushions. Soft foam is good for back cushions or arms. The PFA further qualifies an ILD/IFD rating of 24 to 30 for average seat cushions, 30 to 36 for firmer seat cushions, 10 to 23 for backs and 33 for arms.
Gauge the quality of foam by the density rating, which is the weight of 1 cubic foot of foam. As the density number rises, so does the quality and expense of the foam. The best density levels for furniture upholstery are between 1.8 and 3.2. The numbering may appear as #18, which translates to 1.8 as the density rating. Higher quality foam does better under repeated pressure like you would see on a seat cushion that is used often. Use a density rating of 2.5 or higher for seat cushions. Use foam with a rating of 1.2 to 1.5 for the arms or back of furniture.
Ask if the foam cells are "open," which is the most common type, or "closed." Open cells mean the cells of the foam touch and are open to adjoining cells, making open cell foam suitable for gluing pieces together. With closed cell foam, the cells do not open into one another making this type of foam suitable for boat cushions because of the float capability.
Seek foam that is labeled high resilient (HR), an indication of longer life span. That is particularly beneficial for seat cushions that see a lot of use. In conjunction with HR, for safety purposes, the foam should be fire resistant (FR). HR-FR foam is required by most states for use in a public setting.
Select thickness based on the item being upholstered. Couch and chair cushions should be 4 to 6 inches thick. Dining chair cushions should be 1 to 2 inches thick. Arms and backs can be covered in 1 to 3 inches of foam, cotton batting or a combination.