Lean Body Mass Calculator
Find your lean body mass (everything that isn't fat) from your height and weight, with your fat mass, body fat % and FFMI.
Pick any unit for each, for example feet and inches with kilograms. Results follow your weight unit.
From a DEXA scan, smart scale or the body fat calculator. Lean mass = weight × (1 − body fat %).
Every formula side by side
| Formula | Lean (kg) | Body fat | vs main |
|---|
How your lean body mass is worked out
Your numbers, step by step, with the formula you picked.
- 1
Height, weight, sex
178 cm, 80 kg, man
- 2
Boer formula
32.56 + 47.53
− 19.2 = 60.9 kg - 3
Weight minus lean
80 − 60.9
= 19.1 kg fat - 4
Your lean mass
60.9 kg, 23.9% fat
FFMI 19.2
Read more: every formula, a worked example and notes
Lean body mass (LBM) is your weight minus your fat: muscle, bone, organs, skin, blood and water. It is used to set some medicine and anaesthetic doses, to plan protein and calories, and to track whether weight lost is fat or muscle. Measuring it needs a DEXA scan or similar, so most people estimate it from height, weight and sex with a formula.
General information, not medical advice. Formula results are estimates; very muscular, very lean or very heavy people can be well off.
The formula
Boer (used by default): men LBM = 0.407 × weight in kg + 0.267 × height in cm − 19.2; women LBM = 0.252 × weight in kg + 0.473 × height in cm − 48.3. Fat mass = weight − LBM. Body fat % = fat mass ÷ weight × 100. FFMI = LBM ÷ (height in m)²; normalised FFMI = FFMI + 6.1 × (1.8 − height in m).
The other formulas (weight in kg, height in cm):
| Formula | Men (or all) | Women | Notes |
|---|---|---|---|
| Boer (1984) | 0.407 × kg + 0.267 × cm − 19.2 | 0.252 × kg + 0.473 × cm − 48.3 | the most used for adults; fits a wide range of sizes |
| James (1976) | 1.1 × kg − 128 × (kg ÷ cm)² | 1.07 × kg − 148 × (kg ÷ cm)² | common in drug dosing; breaks down at a high BMI |
| Hume (1966) | 0.32810 × kg + 0.33929 × cm − 29.5336 | 0.29569 × kg + 0.41813 × cm − 43.2933 | from body water studies in adults |
| Peters (2011) | 3.8 × 0.0215 × kg^0.6469 × cm^0.7236 | same for girls and boys | made for children (about 14 and under) |
| Known body fat % | weight × (1 − body fat % ÷ 100) | best when the body fat % is measured | |
Worked example
A man 178 cm tall weighing 80 kg. Boer: 0.407 × 80 = 32.56 and 0.267 × 178 = 47.526, so LBM = 32.56 + 47.526 − 19.2 = 60.886, shown as 60.9 kg (134.2 lb).
Fat mass = 80 − 60.886 = 19.1 kg (19.114). Body fat = 19.114 ÷ 80 × 100 = 23.9%. FFMI = 60.886 ÷ (1.78 × 1.78) = 60.886 ÷ 3.1684 = 19.2; normalised FFMI = 19.217 + 6.1 × (1.8 − 1.78) = 19.217 + 0.122 = 19.3.
For the same man, James gives 1.1 × 80 − 128 × (80 ÷ 178)² = 88 − 128 × 0.20199 = 62.1 kg, and Hume gives 0.32810 × 80 + 0.33929 × 178 − 29.5336 = 26.248 + 60.394 − 29.534 = 57.1 kg. The average of the three adult formulas is (60.886 + 62.145 + 57.108) ÷ 3 = 60.0 kg. If he knew his body fat was 20%, LBM = 80 × 0.8 = 64.0 kg.
Typical results (Boer)
| Example | Height, weight | LBM | Body fat |
|---|---|---|---|
| Woman | 162 cm, 60 kg | 43.4 kg | 27.6% |
| Woman | 170 cm, 75 kg | 51.0 kg | 32.0% |
| Man | 175 cm, 70 kg | 56.0 kg | 20.0% |
| Man | 185 cm, 95 kg | 68.9 kg | 27.5% |
Notes
- James at a high BMI: the James formula subtracts a term that grows with the square of weight, so it gives too low a lean mass for heavy people and, past a BMI of about 43 for men (1.1 × 10,000 ÷ 256) and 36 for women (1.07 × 10,000 ÷ 296), it goes down as weight goes up. Use Boer instead; this page drops James from the average past that point.
- Children: the Boer, James and Hume formulas were made with adults. Peters (2011) was made for children: estimated extracellular fluid = 0.0215 × kg^0.6469 × cm^0.7236, and lean mass = 3.8 × that. Choose "Child" under Advanced.
- FFMI: a general guide only. In a 1995 study (Kouri and others) drug-free trained men rarely went above a normalised FFMI of about 25. Because the LBM here is an estimate, the FFMI is too; for an FFMI from measured body fat, enter your body fat % under Advanced.
- Sources: Boer, Research in Experimental Medicine 1984; James, Research on Obesity (HMSO) 1976; Hume, Journal of Clinical Pathology 1966; Peters and others, Nuclear Medicine Communications 2011; Kouri and others, Clinical Journal of Sport Medicine 1995. Coefficients checked on 13 October 2026.
Common questions
What is lean body mass?
Lean body mass (LBM) is everything in your body that is not fat: muscle, bone, organs, skin, blood and water. It is your weight minus your fat mass. For most adults it is about 60–90% of body weight; men usually have a higher share than women.
How do you calculate lean body mass?
The quickest way is a formula from height, weight and sex. The Boer formula for a man is 0.407 × weight in kg + 0.267 × height in cm − 19.2. For 80 kg and 178 cm that is 32.56 + 47.53 − 19.2 = 60.9 kg. If you know your body fat %, LBM = weight × (1 − body fat %).
Which lean body mass formula is most accurate?
Boer is the one most often used for adults and holds up well across sizes. James is common in medicine dosing but gives results that are too low for people with a high BMI. Hume is close to Boer for most people. For children, Peters is the one made for them. A body fat % from a DEXA scan or a good body fat test beats every formula.
Is lean body mass the same as muscle mass?
No. Muscle is only part of lean mass, roughly half of it in a typical adult. Lean mass also counts bone, organs, skin and body water, so it is always a lot more than your muscle alone.
What is FFMI?
Fat-free mass index (FFMI) is lean mass in kg divided by height in metres squared, like a BMI that counts only lean mass. The normalised FFMI adds 6.1 × (1.8 − height in m) so that short and tall people can be compared. It is a rough guide only, because the lean mass behind it is an estimate.
Why does the James formula give odd results for heavy people?
James subtracts a term that grows with the square of weight. Past a BMI of about 43 for men and 36 for women, adding weight makes the James lean mass go down, which is impossible. This calculator flags James and leaves it out of the average when your BMI is past that point.
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