Lean Body Mass Calculator
Estimate lean body mass with the Boer, James and Hume formulas, or directly from a measured body fat percentage, and see what share of your weight is lean tissue.
Lean Body Mass Calculator: with the default inputs, lean body mass (boer) is 129.5 lb.
- James formula
- 131.5 lb
- Hume formula
- 122 lb
- From your body fat %
- —weight × (1 − body fat %). Shown when you enter a body fat percentage.
- Lean mass as % of weight (Boer)
- 78.5%
- Implied fat mass (Boer)
- 35.5 lb
Assumptions
- Boer is the headline figure; James and Hume are shown for comparison.
- The 'from body fat' line assumes the percentage you enter is accurate — formulas are ignored for it.
- Formulas were fitted to adults of average build; they under-predict lean mass for very muscular people and are unreliable above BMI 35.
- Lean mass (kg)58.7378%
- Fat mass (kg)16.1122%
| Method | Lean mass(lb) | % of weight | Implied body fat |
|---|---|---|---|
| Boer (1984) | 129.5 lb | 78.5% | 21.5% |
| James (1976) | 131.5 lb | 79.7% | 20.3% |
| Hume (1966) | 122 lb | 74% | 26% |
How this is worked out
The formula
Boer (1984): men LBM = 0.407 × W + 0.267 × H − 19.2 · women LBM = 0.252 × W + 0.473 × H − 48.3 James (1976): men LBM = 1.1 × W − 128 × (W ÷ H)² · women LBM = 1.07 × W − 148 × (W ÷ H)² Hume (1966): men LBM = 0.32810 × W + 0.33929 × H − 29.5336 · women LBM = 0.29569 × W + 0.41813 × H − 43.2933 From body fat: LBM = W × (1 − body fat %) W = weight (kg), H = height (cm)
Open How it’s calculated above to see this worked through with your own numbers.
What you enter
- Weight
- A number.in lb, kg, st · from 40 to 1000 · defaults to 165
- Height
- A number.in ft, in, cm, m · from 3 to 9 · defaults to 5.8333
- Sex
- Choose one of 2 options.Male · Female
- Body fat (if measured)(under More options)
- From DEXA, calipers or the body fat calculator. Leave 0 to skip. A measured value beats any formula.a percentage · from 0 to 70 · defaults to 0
What you get back
- Lean body mass (Boer)main answer
- James formula
- Hume formula
- From your body fat %
- weight × (1 − body fat %). Shown when you enter a body fat percentage.
- Lean mass as % of weight (Boer)
- Implied fat mass (Boer)
What this assumes
- Boer is the headline figure; James and Hume are shown for comparison.
- The 'from body fat' line assumes the percentage you enter is accurate — formulas are ignored for it.
- Formulas were fitted to adults of average build; they under-predict lean mass for very muscular people and are unreliable above BMI 35.
About this calculator
Lean body mass is everything you weigh that isn't fat: muscle, bone, organs, water. It's the tissue that sets your resting metabolism, carries your strength, and — in medicine — determines how many drugs distribute. Knowing it turns a scale weight into something you can act on.
Three formulas, one idea
Without a scan you can't measure lean mass directly, so researchers fitted regressions of height and weight against lab measurements:
- Boer (1984) was derived from total body water measurements and is the most widely used today, including in radiology (it scales contrast doses) and pharmacology.
- James (1976) came from a UK Department of Health report on obesity. It works well at normal weights but has a mathematical flaw: at high BMI the quadratic term takes over and predicted lean mass starts to fall as weight rises. Above BMI 35 don't trust it.
- Hume (1966) is the oldest, fitted to a small sample using tracer dilution. It tends to run a little lower than Boer.
For an average-build adult the three agree within 2–4 kg. Where they disagree sharply, it's a sign your build isn't average — and then only a measurement will do.
If you know your body fat percentage
Enter it under More options and the calculator adds the direct estimate: weight × (1 − body fat). A DEXA, Bod Pod, or careful caliper reading beats any formula because formulas assume typical muscularity. A lifter at 180 lb and 12% body fat has about 158 lb of lean mass; Boer would guess around 143.
What to do with the number
- Nutrition: protein targets of 2.0–2.5 g per kg of lean mass are common in physique sports; setting them on lean mass avoids over-prescribing for people carrying more fat.
- Metabolism: the Katch-McArdle BMR formula (370 + 21.6 × LBM) uses lean mass directly.
- Tracking: if you're dieting, aim to lose fat while lean mass holds. Re-estimating every month tells you whether the deficit is too steep.
Lean mass is not the same as muscle mass. Skeletal muscle is roughly 40–50% of lean mass in men and 30–40% in women; the rest is bone, organs and water.
Frequently asked questions
▸What's a normal lean body mass percentage?
Roughly 75–85% of body weight for men and 65–80% for women, the complement of typical body fat ranges. Athletes sit higher; the formulas here assume average builds.
▸Which lean body mass formula is best?
Boer is the usual default and holds up best across body sizes. James fails at high BMI. If you have a measured body fat percentage, use that instead of any formula.
▸Is lean body mass the same as muscle mass?
No. Lean mass includes bone, organs, skin and water. Skeletal muscle is typically 40–50% of lean mass in men and 30–40% in women.
▸Why does lean mass matter for calories?
Lean tissue is metabolically active; fat is not, much. Two people at the same weight but 10 kg apart in lean mass can differ by 200+ kcal a day at rest.
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