What lean body mass is — and is not
Lean body mass (LBM) is everything in your body that is not fat: muscle, bone, organs, skin, blood and water. It is the metabolically active, structurally essential part of you, and it matters for practical reasons. Calorie and protein needs scale with lean mass more than with total weight; medication doses are sometimes calculated from it; and for anyone dieting, the real goal is almost never "lose weight" but "lose fat while keeping lean mass" — two people can lose the same ten kilograms with completely different results depending on how much of it was lean. This calculator estimates LBM three different ways so you can see both a central figure and the honest disagreement between methods.
The Boer, James and Hume formulas
All three equations were fitted against measured body composition in reference groups, using only weight (W, kg) and height (H, cm). The Boer formula (1984) is the most widely used in clinical practice and is this page's headline. The James formula takes a different shape — it penalises a high weight-for-height ratio quadratically — and tends to drift at very high weights. The Hume formula (1966), the oldest, was derived from body-water measurements. Women's versions of each appear automatically when you select female; they are printed in full in the explanation under the calculator in the original papers and follow the same structure with different constants.
How to read three different answers
Beginners expect one true number; these formulas honestly give you a range. They disagree because each was fitted on different people with different measurement methods, and because weight and height alone cannot distinguish a muscular 90 kg from a soft 90 kg — the formulas return the lean mass of an average person of your size and sex. If the three results cluster within a couple of kilograms, the estimate is probably useful for planning protein or calories. If they spread widely, that is the formulas telling you your build is far from average for your size, and a direct method will serve you better: the U.S. Navy tape method on the Body Fat Calculator, skinfold callipers used by someone experienced, or a DEXA or bio-impedance scan, each imperfect in its own documented ways.
Using lean mass well
The two most useful things to do with an LBM figure are setting protein and judging a diet. Protein targets for preserving muscle while losing fat are commonly expressed per kilogram of lean mass (or per kilogram of goal weight, a close cousin), which avoids the absurd targets that per-total-weight rules give people carrying a lot of fat. And during weight loss, re-estimating LBM as you go shows what you are actually losing: if weight falls ten kilograms while lean mass falls only one or two, the diet is working beautifully even if the pace feels slow. Expect day-to-day noise — lean mass includes body water, so hydration, glycogen and a salty meal can move any measurement — and judge by monthly trends, never single readings.
Worked example
An 80 kg man, 180 cm tall. Boer: 0.407 × 80 + 0.267 × 180 − 19.2 = 32.56 + 48.06 − 19.2 = 61.42 kg lean. James: 1.1 × 80 − 128 × (80 ÷ 180)² ≈ 62.72 kg. Hume: 0.32810 × 80 + 0.33929 × 180 − 29.5336 ≈ 57.79 kg. On the Boer figure, fat mass is 80 − 61.42 = 18.58 kg, and lean mass is about 76.8% of his weight.
How to use this calculator
- Select male or female — each formula has a separate version for each sex.
- Enter your weight in kilograms and your height in centimetres.
- Read the Boer estimate in the headline panel, with James and Hume beside it in the breakdown.
- Check the fat mass and lean percentage in the stats — they are the Boer figure subtracted from, and divided into, your weight.
- If the three formulas disagree widely, follow up with a measurement-based method such as the Body Fat Calculator.
Lean body mass vs fat-free mass
One terminology note, because the two are often used as if identical: fat-free mass excludes all fat, including the small amount of essential lipid inside cell membranes, bone marrow and the nervous system, while lean body mass traditionally includes that essential fat. In practice the numbers differ by only a few percent and most consumer tools — this one included — use the terms loosely. What matters is comparing like with like over time: use the same formula, or the same device, under similar conditions, and the trend it shows you will be meaningful even if the absolute level carries a margin of error.
Finally, a word of proportion. Lean mass changes slowly — a beginner training hard and eating well might add a few kilograms of muscle in a year, and preserving lean mass through a diet is a realistic triumph rather than a consolation prize. Any tool or product promising rapid lean-mass swings is almost always measuring water. Set the plan by the slow, real physiology and the formulas on this page will track it honestly.
Important note
For information only — not medical advice. This calculator uses standard published formulas to estimate values for general education and planning. It does not diagnose, treat, or replace the advice of a doctor or other qualified health professional. If you have a health concern, are pregnant, or are making decisions about treatment, medication, or a major diet or exercise change, please talk to a healthcare professional first.