A single liter of human fat weighs approximately 900 grams or 1.9 pounds. According to Nutracheck, the average density of human fat is .9 grams per milliliter.
The substance commonly referred to as fat is technically adipose tissue. Adipose tissue is composed of a network of cellular membranes and fibers that contain globules of fat. There are two types of adipose tissue: white and brown. White adipose tissue, the most common, is used for insulation, cushioning and energy. This type of fat helps retain body heat, which is especially important in cold climates. It reduces trauma to internal organs and structures in much the same way as bubble wrap in a package. It is also the densest form of energy storage, able to store 9 calories per gram, whereas carbohydrates and proteins can only store 4 calories per gram. Brown adipose tissue is found mainly in young mammals and mammals who hibernate. This type of fat actually consumes energy to produce heat.