A memory foam mattress is usually denser than other foam mattresses, making it both more supportive and heavier. Memory foam mattresses are often sold for higher prices than traditional mattresses. Memory foam used in mattresses is commonly manufactured in densities ranging from less than 24kg/m (1.5 lb/ft) to 128kg/m (8 lb/ft) density. Most standard memory foam has a density of 16–80 kg/m (1 to 5 lb/ft). Most bedding, such as topper pads and comfort layers in mattresses, has a density of 48–72 kg/m (3 to 4.5 lb/ft). High densities such as 85 kg/m (5.3 lb/ft) are used infrequently.
The firmness property (hard to soft) of memory foam is used in determining comfort. It is measured by a foams indentation force deflection (IFD) rating. However, it is not a complete measurement of a "soft" or "firm" feel'. A foam of higher IFD but lower density can feel soft when compressed.
IFD measures the force in newtons (or pounds-force) required to make a dent 1 inch into a foam sample  by a 323 cm (50 sq in, 8-inch-diameter) disc—known as IFD @ 25% compression. IFD ratings for memory foams range between super soft (IFD 10) and semi-rigid (IFD 12). Most memory foam mattresses are firm (IFD 12 to IFD 16).
Second and third generation memory foams have an open-cell structure that reacts to body heat and weight by molding to the sleepers body, helping relieve pressure points, preventing pressure sores, etc. Manufacturers claim that this may help relieve pressure points to relieve pain and promote more restful sleep, although there are no objective studies supporting the mattresses claimed benefits.
Memory foam mattresses retain body heat, so they can be excessively warm in hot weather.  However, gel-type memory foams tend to be cooler due to their greater breathability.