Know what each number means

Understand Your Body Composition

Body weight, body fat and lean mass answer different questions. Choose a metric for the decision you need to make.

Separate the terms

Body weight is total mass. Fat mass is the estimated mass of body fat. Fat-free mass includes everything else, including water, bone and muscle; it is not a synonym for skeletal muscle.

  1. Body-fat percentage = fat mass ÷ body weight × 100.
  2. At 80 kg and an estimated 20% body fat, estimated fat mass is 16 kg and fat-free mass is 64 kg.
  3. An uncertain body-fat estimate makes both derived mass estimates uncertain.

Choose your next question

Follow the method-specific guide instead of comparing unlike measurements.

Understand body-fat estimates

Choose between a visual estimate, tape method and reference measurement.

Calculate FFMI

Put estimated fat-free mass in the context of height.

Photo-based composition

See which outputs can be estimated and which cannot be measured from a photo.

Common questions

Is lean mass the same as muscle mass?

No. Fat-free mass includes water, bone, organs and other non-fat tissues as well as muscle. A change in this number does not prove an equal change in muscle.