The two most important considerations in selecting the kind of foam for rocking chair cushions are the firmness and the density. They are NOT the same thing, and you may end up very dissatisfied with the results if you don’t know the difference. This applies whether you are buying ready-made cushions, having custom cushions made 
up by an upholsterer, or making them up yourself.
Firmness is based on how much you sink in when you sit on the foam cushion. It is measured by the amount of weight necessary to compress it to a certain percentage of its original volume. Density indicates how much material surrounds the air in the foam, and is measured by the weight for a specific volume of the foam.
When the specifications for firmness of foam are given, they are usually the first 2 numbers in a group of 4, while the specifications for density are given in the second two numbers. Firmness usually runs from about 12 (for very soft) to 55 (for very firm). Density usually runs from about 10 to 25 for ordinary foam, and about 35 to 55 for “memory foam”.
But what does this mean for practical purposes? A heavy person needs a firmer cushion to be comfortable, otherwise the cushion is mashed flat and there is no support. A child needs a soft rocking chair cushion; otherwise the cushion is as stiff as the seat itself. Density has more to do with how long the cushion keeps its shape. Denser cushions will generally last longer if they’re exposed to the same treatment as less dense ones. The same is true of memory foam, which also has different degrees of density, even though it may be very soft. One test of memory foam is how long it takes to spring back to its original shape after being pressed down. The denser the foam, the longer it takes to return to its original shape.
Because there is more material and less air in denser foam, expect to pay more for it. In the long run you may save, however, because it will need to be replaced less often.