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Lean Body Mass Calculator
Use this free lean body mass calculator to split your weight into lean mass and fat mass. Enter your sex, height, and weight, and it estimates lean body mass with the Boer formula, then works out your body fat percentage, fat mass, and a daily protein target based on lean mass.
Lean body mass includes muscle, bone, water, and organs — everything that is not fat. It is the number dietitians use to set protein targets.
Your body composition
The bar is your total weight split into lean mass (blue) and fat mass (amber). The proportions come from the Boer formula below.
How it works
Your weight on a scale is really two numbers hiding inside one: the lean part and the fat part. Lean body mass is everything that is not fat — muscle, bone, water, organs, and skin. Knowing it matters, because two people can weigh exactly the same and be in very different shape. A 85 kg athlete and a 85 kg office worker have the same scale weight but very different amounts of muscle, and they need different diets, different training, and different expectations.
This lean body mass calculator estimates your lean mass from your sex, height, and weight using the Boer formula. It was developed by comparing simple measurements against precise laboratory scans, and it is one of the most widely used formulas in nutrition and medicine because it needs no special equipment — just a tape measure and a scale. Separate formulas exist for men and women, because men and women carry different proportions of muscle and fat at the same height and weight.
Women: LBM = 0.252 × weight (kg) + 0.473 × height (cm) − 48.3
Once lean mass is known, the rest follows by subtraction. Fat mass is simply total weight minus lean mass, and body fat percentage is fat mass divided by total weight, times 100. That percentage is the number most people actually care about, and it is a much better measure of fitness than weight alone or even body mass index, because it separates the muscle from the fat instead of lumping them together.
What counts as a healthy body fat percentage depends on sex and age. For men, a common reference range is about 10 to 20% for fit adults, with athletes often below 15% and average adults between 18 and 25%. For women, healthy ranges run higher because women naturally carry more essential fat: roughly 18 to 28% for fit adults, with athletes near 14 to 20%.
Lean mass is also the number to use when setting protein intake. Sports nutrition guidance usually suggests about 1.6 to 2.2 grams of protein per kilogram of lean body mass per day for people who train, which is more accurate than basing protein on total weight, because fat tissue does not need protein the way muscle does. For the example on this page — a man with 63.5 kg of lean mass — that works out to roughly 102 to 140 grams of protein a day. If you are tracking fat loss, watch lean mass as well as weight: the goal is for the scale to drop while lean mass stays steady, which means the loss is coming from fat rather than muscle.
Be honest about the limits. The Boer formula is an estimate built from population averages, and it is less accurate for people far outside the average — very muscular lifters, very tall or short people, and people with a lot of body fat can all get results that are off by several kilograms. Treat the result as a useful starting point for tracking change over time, not as a laboratory measurement.
Worked example
Take a man who is 180 cm tall and weighs 85 kg. Here is exactly what the calculator does with those numbers.
Step 1 — Lean body mass, using the men's Boer formula: 0.407 × 85 = 34.60, plus 0.267 × 180 = 48.06, minus 19.2. That gives 34.60 + 48.06 − 19.2 = 63.5 kg of lean mass.
Step 2 — Fat mass. Subtract lean mass from total weight: 85 − 63.5 = 21.5 kg of fat.
Step 3 — Body fat percentage. Divide fat mass by total weight and multiply by 100: 21.5 ÷ 85 × 100 = 25.3% body fat.
Step 4 — What it means. For a man, 25.3% is above the fit range of about 10 to 20% and at the upper end of average. His protein target at 1.6 to 2.2 g per kg of lean mass would be 63.5 × 1.6 = 102 g to 63.5 × 2.2 = 140 g per day.
Step 5 — Compare a woman. A woman who is 165 cm tall and weighs 60 kg: 0.252 × 60 = 15.12, plus 0.473 × 165 = 78.05, minus 48.3, giving 44.9 kg of lean mass, 15.1 kg of fat, and 25.2% body fat. Enter the man's numbers — male, 180 cm, 85 kg — and the tool shows exactly 63.5 kg lean mass, 21.5 kg fat, and 25.3% body fat, with the composition bar split to match.
FAQ
What is the difference between lean body mass and muscle mass?
Lean body mass is everything that is not fat: muscle plus bone, water, organs, and skin. Muscle mass is only the muscle part, which is roughly half of lean mass. This calculator gives lean body mass, which is the standard figure for nutrition and drug dosing, because it is the part of the body that actively burns energy.
Why does the Boer formula use different equations for men and women?
Men and women have different typical body compositions at the same height and weight — men carry more muscle and less fat on average, while women carry more essential fat. A single formula would overestimate lean mass for women and underestimate it for men, so the Boer formula fits a separate equation to each sex.
Is lean body mass the same as fat-free mass?
For practical purposes, yes. The two terms are used almost interchangeably outside a laboratory. Strictly, fat-free mass includes a tiny amount of essential fat in the brain and organs, while lean body mass sometimes excludes it, but the difference is a fraction of a kilogram and does not change any decision you would make from the result.
How accurate is this calculator?
For an average adult it is usually within a few kilograms of a laboratory scan, which is good enough for tracking trends and setting protein targets. It is less accurate for very muscular people, very tall or short people, and people with high body fat, because the formula was fitted to average bodies. If you need a precise number, a DEXA scan is the reference method.
Should I lose weight or gain muscle first?
If your body fat percentage is high — roughly above 25% for men or 32% for women — most coaches suggest a slow fat loss phase first, while keeping protein high and lifting weights to protect lean mass. If your body fat is already low and your lean mass is modest, a slow muscle-building phase makes more sense. Either way, track lean mass, not just scale weight.