Memory foam comes in a wide variety of densities, and we often get asked what this density number means and how to compare memory foams.
First, what is density. This is the weight of a memory foams, and tells you a bit about it. The higher the number, the more material is in the foam, and so the more cushion as a general rule
However, in my experience, as the density gets higher -- above 4 lb/cubic foot (or 4 lb from here on out), the memory foam gets stiffer feeling. A 5 lb memory foam can have a nice feel, but I generally recommend a 4 lb memory foam as the top layer in a bed or in a topper since it offers the sweet spot of a soft conforming feel and a nice amount of cushion.
What does a 4 lb/cubic foot density mean? It just means that a cube of this foam that is a foot in each dimension weighs 4 lbs.
This measure of density doesn't give you any sense of the firmness of the foam. This is measured by another number called either the ILD or IFD, which measures how much pressure it takes to compress this foam a certain amount. The lower the number the softer the foam (since it takes a lesser amount of pressure to compress it is a lower number).
Memory foams are, in general soft foams, and the 4 lb memory foam I sold typically had ILD's of around 10 - 15.
These two numbers (density and ILD) alone, though, don't tell the whole story on a memory foam.
Two memory foams with identical numbers can feel very different to sleep on since other issues are how a memory foam responds to temperature. Memory foams are what are called visco-elastic foams, and that means they are viscous at colder temps (so firmer) and more elastic at warmer temps (so softer). But each memory foam has its own characteristics, and so one memory foam could be firmer at typical home temps and one with a different temp responsiveness could be much softer.
One example of this is the classic Tempur-Pedic (tm) Tempur (tm) foam which can be pretty firm at room temps that fall below 70 degrees where as the topper foam I've carried was generally soft at these temps so it was more user friendly.
This is important, so try to research this through the reviews you find on-line for the particular memory foam mattress or topper you are interested in.
Another issue is how hot the memory foam sleeps -- or another way of looking at it is the air flow of the memory foam. Latest generation memory foams, which are often more expensive than standard memory foams or the cheap memory foam from China, has much better air flow -- some claim 95+% better air flow. And this translates to cooler sleep. In my own experience with this with the last topper we carried, we got almost no complaints of it sleeping hot or having an odor, and I attribute this to it being this more expensive memory foam offering much better air flow.
So when you are shopping for memory foam, do take density into account, but density isn't an end all/be all and it is important to also look at other characteristics like the firmness of the memory foam (ILD) as well as it temperature sensitivity and also its air flow characteristics.