About 3,500 calories are conventionally associated with storing one pound of body fat.
This figure comes from estimating the energy contained in adipose tissue, the body’s main long-term energy store. It became popular as the “3,500-calorie rule”: a sustained energy surplus of roughly 3,500 calories was traditionally expected to produce one pound of weight gain, while an equivalent deficit was expected to produce one pound of loss.
The rule is a useful approximation, not a precise biological law. Body weight also includes water, glycogen, muscle, connective tissue, and undigested food. Metabolism can change during weight loss or gain, and the body does not convert every excess calorie directly into pure fat. Dietary fat itself contains about 9 calories per gram, but stored body fat includes other components, which helps explain why the practical estimate differs from a simple conversion of 454 grams of pure fat.
Even with those qualifications, “one pound” is the standard answer to this classic nutrition question.