How this lean body mass calculator works
Your lean body mass (LBM) is the part of your total weight that is not fat — muscle, bone, organs, connective tissue and water. It is simply your total weight minus your fat mass. Tracking LBM gives a clearer picture of body composition than weight alone, because two people of the same weight can carry very different amounts of muscle and fat.
The most accurate way to find LBM is to start from a measured body fat percentage: LBM = weight × (1 − body fat % ÷ 100). When you do not have that number, validated prediction equations estimate LBM from your sex, weight and height. This tool shows three common ones side by side so you can compare:
Boer — men: 0.407 × kg + 0.267 × cm − 19.2; women: 0.252 × kg + 0.473 × cm − 48.3. James — men: 1.1 × kg − 128 × (kg ÷ cm)²; women: 1.07 × kg − 148 × (kg ÷ cm)². Hume — men: 0.32810 × kg + 0.33929 × cm − 29.5336; women: 0.29569 × kg + 0.41813 × cm − 43.2933. Imperial inputs are converted first (lb × 0.453592, in × 2.54).
Reference note: fat mass is your total weight minus the lean body mass shown. The headline result uses your body fat percentage when you enter one (most accurate) and otherwise the Boer estimate. These prediction equations are population averages and can differ for very lean, very muscular or higher-weight individuals.
Frequently asked questions
- What is lean body mass?
- Lean body mass (LBM) is the total weight of everything in your body that is not fat — muscle, bone, organs, connective tissue and water. It is your total weight minus your fat mass, and it tracks the muscle and other active tissue that body composition goals usually focus on.
- How is lean body mass calculated?
- When body fat percentage is known, LBM is weight × (1 − fat fraction), which is the most accurate approach. When it is unknown, validated equations estimate it from your sex, weight and height. This calculator shows three — Boer, James and Hume — side by side so you can compare.
- Which lean body mass formula is best?
- There is no single best formula for everyone. The Boer equation is widely used and works well for average adults; James and Hume are long-standing alternatives that can differ for very lean, very muscular or higher-weight people. Entering a measured body fat percentage gives the most accurate result of all.
- What is the difference between lean body mass and fat-free mass?
- The terms are often used interchangeably. Strictly, fat-free mass excludes all fat, while lean body mass includes the small amount of essential fat in organs, bone marrow and the nervous system. The difference is only a few percent, so the estimates here are treated as approximations of both.
- Why does lean body mass matter?
- Lean body mass reflects muscle and other active tissue, so it relates to strength, metabolic rate and overall body composition. Tracking it alongside total weight helps you tell whether changes come from fat or muscle — useful when dieting, training or maintaining muscle with age.
- Should I consult a professional about my lean body mass?
- Yes if you have concerns. This calculator is an educational estimate and does not provide diagnosis or treatment; these figures are informational and not medical advice. If you are planning major weight or training changes or have a health condition, consult a qualified doctor, dietitian or other health professional before acting on any number here.