First off I just want to say that choosing foam doesn't have to be as complicated as people make it. Most of the time you will be just fine with a medium or firm density. Usually your supply store will only have 3-4 options for you to choose from anyways. I typically just go off of feel for what I want. You can also assume that once the foam is covered it will be a bit firmer so take that in to consideration.
Upholstery foam has two ratings that are important to understand when choosing foam for your project, density and firmness. Density is how much the foam weighs per cubic foot. If foam has a density of 28 this means that it weighs 2.8
pounds per cubic foot. Firmness is how much weight it takes to compress the foam 1/3. If foam has a firmness of 25 this means that it takes 25 pounds to compress the foam 1/3. When shopping for foam these ratings are usually represented with density first then firmness. Example = 15/30 or 1530 this would represent a foam with a density of 1.5 pounds and firmness of 30 pounds. When choosing foam for your project the most important consideration is the density. The higher the density the longer the foam will last before it starts to loose shape and firmness. The firmness is nice to know but your feel is the best way to judge what is best for your project. Also keep in mind when choosing foam that once you make and install your cover on the foam it will feel more firm than with no cover. Knowing this it is better to use a little softer foam to start with.
Foam is also available in marine grade and outdoor foam. Marine grade foam is mold resistant and is used mostly in boats. Outdoor foam is very porous and does not hold water so it dries much faster than normal foam, witch can take days to
dry out completely.
Hope this helps guys!