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Lean Body Mass Calculator
Sign up to favourite this calculatorEnter your height, weight and sex to compare estimated lean body mass from the Boer, James and Hume equations. The calculator shows the range and average of these formula estimates, plus formula-estimated fat mass and the estimated fat-free share of total body weight.
Estimated lean body mass range
Range across the Boer, James and Hume formula estimates.
Average of the three equations
60.0 kgArithmetic average; not a direct measurement.Difference across equations
5.0 kgHighest minus lowest estimate.Individual equation estimates
| Equation | Estimated lean body mass |
|---|---|
| Boer (1984) | 60.9 kg |
| James (1976) | 62.1 kg |
| Hume (1966) | 57.1 kg |
Different published equations can produce different estimates from the same height, weight and sex.
Formula-estimated fat mass
20.0 kgEstimated fat-free share of total weight
75.1%These values are derived from the three formula estimates; they are not direct body-composition measurements.
These equations produced an implausible estimate for these measurements. Check the inputs; formula-based lean-body-mass estimates may be unreliable at some body sizes.
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Assumptions and limits
- An estimate from the numbers you entered, for guidance only. It is not financial, tax or medical advice.
- Calculator version 1, last updated 25 October 2026.
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How to use this calculator
- Pick your sex — every equation here uses different constants for men and women.
- Enter your height, and pick its unit — centimetres, metres, feet, inches, or feet and inches.
- Enter your weight, and pick its unit — kilograms, pounds, or stones and pounds.
- Read the estimated lean body mass range across the three equations, their arithmetic average, the difference between the highest and lowest estimate, each equation's own figure, the formula-estimated fat mass, and the estimated fat-free share of your total weight.
Nothing is submitted anywhere. The calculation runs in the page as you type, and the link in your address bar carries your figures, so it can be shared or bookmarked.
What do I need to enter?
- Sex — Male or Female. Every equation uses different constants for each.
- Height — from 130 cm to 220 cm, or the equivalent in metres, feet, inches or feet and inches (the inches box takes 0 to 11).
- Weight — from 30 kg to 300 kg, or the equivalent in pounds or stones and pounds (the pounds box takes 0 to 13). It feeds the equations and the fat-mass figures.
What do the results show?
- Estimated lean body mass range — the main figure: the lowest to the highest of the Boer, James and Hume estimates.
- Average of the three equations andDifference across equations — the arithmetic mean of the three, and the gap between the highest and lowest.
- Individual equation estimates — a table with each equation's own figure.
- Formula-estimated fat mass — your weight minus the average lean body mass.
- Estimated fat-free share of total weight — the average lean body mass as a percentage of your weight.
How the estimates are calculated
The calculator converts all inputs to kilograms and centimetres, then calculates three published lean-body-mass estimates. W is weight in kilograms and H is height in centimetres.
| Equation | Male | Female |
|---|---|---|
| Boer (1984) | 0.407 × W + 0.267 × H − 19.2 | 0.252 × W + 0.473 × H − 48.3 |
| James (1976) | 1.10 × W − 128 × (W / H)² | 1.07 × W − 148 × (W / H)² |
| Hume (1966) | 0.32810 × W + 0.33929 × H − 29.5336 | 0.29569 × W + 0.41813 × H − 43.2933 |
Average = (Boer + James + Hume) ÷ 3
Difference across equations = highest − lowest
Estimated fat mass = W − average
Fat-free share = average ÷ W × 100
- W is body weight in kilograms; the equations above use H in cm.
- Each equation is worked out separately, so the range shows their disagreement.
“Lean body mass” is used here as a formula-estimated fat-free portion of total body weight. It includes more than skeletal muscle, such as water, bone, organs and connective tissue. The equations can produce different answers because they use different mathematical models and were developed in different populations.
The range and difference across equations show how much the formula estimates vary for the measurements entered. Neither the range nor the average is a direct body-composition measurement.
Worked example. For a 178 cm, 80 kg man the Boer estimate is 0.407 × 80 + 0.267 × 178 − 19.2 ≈ 60.9 kg, James ≈ 62.1 kg and Hume ≈ 57.1 kg — a range of 57.1–62.1 kg, an average of 60.0 kg and a difference across equations of 5.0 kg.
Lean body mass vs. body fat percentage vs. FFMI
The body fat percentage calculator estimates body fat directly from tape measurements and derives lean mass from that. This calculator works the other way — estimating lean mass from height and weight, with fat mass as the remainder. Once you have a lean or fat-free mass figure, the FFMI calculator turns it into an index adjusted for height, useful for comparing muscularity across different heights.
Limitations of these equations
- Each equation is a population-average estimate from height, weight and sex, not a direct measurement of your body composition, muscle, bone or body water.
- Estimates can differ materially between equations, and may be less accurate for people with unusually high muscularity, substantial fluid retention or loss, pregnancy, older age, or very low or high body weight.
- Designed for adults. Not validated for children, teenagers or pregnancy.
- It is not a diagnosis, a body scan, a fitness assessment or medical advice.
Frequently asked questions
What is a lean body mass calculator?
It estimates lean body mass — the fat-free portion of total body weight, covering muscle, bone, organs and water — from your height, weight and sex. It runs three published equations (Boer, James and Hume), then shows the range across them, their arithmetic average, the difference between the highest and lowest estimate, each equation on its own, and the fat mass and fat-free share that follow from the average.
Why does the calculator show a range instead of one number?
Boer, James and Hume are different mathematical models built on different study populations, so they rarely agree exactly. The range — the lowest to the highest of the three estimates — shows how much uncertainty there is in estimating lean mass from height and weight alone. A single averaged figure hides that spread.
What is the average of the three equations?
It is simply the arithmetic mean of the Boer, James and Hume estimates: add the three figures and divide by three. It is supplementary information, not a validated composite measurement, and it is not more accurate than the individual equations — it just sits in the middle of them.
What does the difference across equations tell me?
It is the highest estimate minus the lowest. A small difference means the three equations broadly agree for your measurements; a larger one means the estimate is more model-dependent and should be treated with more caution.
Is lean body mass the same as muscle mass?
No. Lean body mass here means the formula-estimated fat-free portion of total body weight. That includes skeletal muscle but also water, bone, organs and connective tissue, so it is always larger than muscle mass. These equations do not measure skeletal muscle.
Which lean body mass formula is most accurate?
There is no single best equation for every person. Boer (1984) is often cited as the closest match to body-density measurements and is listed first here, but accuracy varies with body size, sex and how similar you are to the populations the equations were developed on. That is why all three are shown side by side rather than collapsed into one.
Is this a body composition or health assessment?
No. These are population-based formula estimates, not a scan of your body and not a diagnosis, fitness assessment, nutritional assessment or personalised target. For a direct body-composition measurement, a qualified clinician or service may use an appropriate validated method such as DEXA or bioelectrical impedance, depending on the purpose.
Last updated 25 October 2026.