Density measures the weight of one cubic foot of memory foam. This is determined by how tightly the foam is compacted. Generally, memory foam densities are rated as follows:Low: 3.5 pounds and underMedium: 3.5 to 5 poundsHigh: 5 pounds and overHigh-density memory foam conforms more closely to contours, provides more support, and is by far the most durable, but it's also more sensitive to heat, so keep this in mind if you sleep hot. Low-density memory foam, on the other hand, typically isn't as supportive, but it might be more comfortable for users who overheat easily and don't necessarily need a highly supportive pillow. Medium-density memory foam strikes a good balance between the two in terms of support, breathability, comfort, and durability.