How to use the lean body mass calculator
- Choose Metric (cm, kg) or US units (ft, in, lb).
- Pick your sex. Each formula has separate versions for men and women.
- Enter your height and weight.
- Choose which formula drives the main result. The table always shows all three.
How lean body mass is calculated
Lean body mass (LBM) is your total weight minus your fat. These three formulas estimate it from weight in kilograms and height in centimeters:
W is weight in kilograms and H is height in centimeters. Boer and Hume are straight-line fits: more height or more weight always means more lean mass. James adds a penalty that grows with weight relative to height, which works for average builds but breaks down at high body weights. The calculator uses Boer by default because it gives sensible results across a wide range of body sizes.
Worked example
These are the calculator's defaults: a man, 175 cm tall, weighing 70 kg.
- Boer: 0.407 × 70 = 28.49, and 0.267 × 175 = 46.73, so LBM = 28.49 + 46.73 − 19.2 = 56.0 kg
- Fat mass: 70 − 56.0 = 14.0 kg, an implied body fat of 14.0 ÷ 70 = 20.0%
- James: 1.1 × 70 = 77, and (70 ÷ 175)² = 0.16, so LBM = 77 − 128 × 0.16 = 56.5 kg
- Hume: 0.32810 × 70 + 0.33929 × 175 − 29.5336 = 22.97 + 59.38 − 29.53 = 52.8 kg
Boer and James agree closely here. Hume gives about 3 kg less, which is typical: it implies a body fat around 24.6% for the same person.
Why lean mass matters
Lean mass drives a large part of how many calories you burn at rest, so some BMR formulas use it directly (the Katch-McArdle equation in the BMR calculator). Doctors use estimates like these to dose some medicines, especially anesthetics and drugs that don't dissolve in fat. And lean mass is a better base than total weight for planning protein intake when someone carries a lot of body fat.
Limits of these formulas
All three were fitted to groups of ordinary adults, so they describe an average person of your height and weight, not you. They can't see muscle, so a strength athlete and an inactive person of the same size get the same answer. The James formula underestimates lean mass at a BMI of about 35 or more. None of the formulas is meant for children, during pregnancy, or for people with large fluid shifts, such as heart or kidney failure. For a measurement rather than an estimate, a tape-measure method such as the body fat calculator, or a DEXA scan arranged through your doctor, will be closer.
Frequently asked questions
What is the difference between lean body mass and fat-free mass?
They are close but not identical. Fat-free mass excludes all fat. Lean body mass, strictly defined, includes the small amount of essential fat found in nerves, bone marrow and organs. In everyday use, and in these formulas, the two terms mean nearly the same thing.
Which lean body mass formula should I use?
Boer is a good default for adults because it stays sensible across most body sizes. James works well for average builds but not at high weights. Hume tends to give lower values. Comparing all three shows how much uncertainty there is.
Can I increase my lean body mass?
Yes. Regular strength training plus enough protein builds muscle over months, and that raises lean mass. These formulas won't show the change, though, because they only use height and weight. Track it with tape measurements or a body composition scan instead.
Is a higher lean body mass always better?
Generally more muscle supports strength, balance and metabolic health, especially with age. But the number itself isn't a goal. If you have questions about body composition and your health, talk to a doctor or a registered dietitian.
Related calculators
- Body Fat Calculator — Estimate your body fat percentage with the U.S. Navy tape method and compare it with a BMI-based estimate.
- BMR Calculator — Your basal metabolic rate from the Mifflin-St Jeor, revised Harris-Benedict and Katch-McArdle equations.
- Protein Calculator — Daily protein in grams from your body weight, from the RDA minimum to targets for older adults and people who train.
- Ideal Weight Calculator — Ideal body weight from the Robinson, Miller, Devine and Hamwi formulas, next to the healthy BMI range.
Results are estimates for general information and are not medical advice. Talk to a doctor or other qualified professional about your health. See our disclaimer.