Lean body mass
calculator
Your lean body mass — everything that isn't fat — estimated from height, weight, and sex by the three formulas people actually cite: Boer, James, and Hume, side by side with the range they span. Plus two things most LBM calculators leave out: a warning when the James formula runs backwards, and the direct body-fat method that beats all three when you have the number. Imperial or metric.
Lean body mass is what you'd weigh with no body fat — muscle, bone, organs, connective tissue, and the water in them. You can't measure it from height and weight alone, but three equations estimate it: Boer (1984), James (1976), and Hume (1966). For a 180 lb (81.65 kg) man at 5'10" (178 cm) they land between 126.9 and 138.5 lb (57.6–62.8 kg) — a spread of about 11.6 lb (5.2 kg) that comes entirely from which equation you pick. If you actually know your body-fat percentage, skip all three: lean body mass is just weight × (1 − body fat), and that number is closer to the truth than any height-based estimate.
How the math works
Each formula takes weight in kilograms and height in centimetres and returns lean body mass in kilograms, with separate coefficients for men and women. Boer and Hume are linear — a fixed fraction of weight plus a fraction of height minus a constant. James is different: it subtracts a term that grows with the square of weight. Written out:
The calculator converts pounds and feet-inches to kilograms and centimetres with the exact 1 lb = 0.45359237 kg and 1 in = 2.54 cm factors, runs each formula, and shows the three results plus the range they span. It also prints the body-fat percentage each estimate implies — 1 − LBM/weight — so you can see what composition the formula quietly assumes for your size.
The three equations came from different reference cohorts measured by different methods (total body water, densitometry) in the 1960s–80s, which is why they disagree. None was built as a fitness tool; Boer's came from a paper on normalising body-fluid volumes, and James's from a UK government obesity report. That a 2026 fitness page still runs mid-century regression equations says more about how rarely these calculators get revisited than about the equations.
The three formulas at a glance
| Formula | Year | Shape | Origin |
|---|---|---|---|
| Boer | 1984 | Linear in weight | Body-fluid normalisation (Am J Physiol) |
| James | 1976 | Quadratic in weight | DHSS/MRC obesity report (HMSO) |
| Hume | 1966 | Linear in weight | Body-composition prediction (J Clin Pathol) |
Worked example
Take a 5'10" (178 cm; 70 in) man at 180 lb (81.65 kg). Convert to metric, then run each formula:
- Boer: 0.407 × 81.65 + 0.267 × 177.8 − 19.2 = 61.5 kg (135.6 lb) — implies about 25% body fat.
- James: 1.1 × 81.65 − 128 × (81.65 / 177.8)² = 62.8 kg (138.5 lb) — implies about 23% body fat.
- Hume: 0.32810 × 81.65 + 0.33929 × 177.8 − 29.5336 = 57.6 kg (126.9 lb) — implies about 29% body fat.
- Range across the three: 126.9–138.5 lb (57.6–62.8 kg); a spread of 11.6 lb (5.2 kg), Hume low and James high.
Run a 5'5" (165 cm) woman at 150 lb (68.04 kg) and the shape holds but tighter: Boer 103.5 lb, James 105.1 lb, Hume 101.1 lb (45.9–47.7 kg), a spread of just 4.0 lb (1.8 kg). The disagreement between formulas is the honest part — no one of these is "your" lean body mass, and printing any of them to a tenth of a pound is more precision than the method can carry.
Now the direct check. If that 180 lb man is a lean 12% body fat, his real lean body mass is 180 × (1 − 0.12) = 158.4 lb (71.8 kg) — about 20 lb above what Boer returns. The formulas didn't fail; they never saw that he was lean. That gap is the whole story of when these estimates are wrong.
When the James formula runs backwards
The James equation is the one to watch, because it's quadratic in weight. At a fixed height, its predicted lean mass rises with weight, reaches a peak, and then falls — so past a certain point, adding weight makes the formula report less lean mass, which is impossible. The peak is where the slope hits zero: for men it lands at a body mass index of about 43 (independent of height), and for women about 36. For a 5'10" man that peak is around 300 lb (136 kg); above it, James is no longer estimating anything real.
This isn't a rounding quirk. It's why the James lean-body-weight formula has been shown unsuitable for weight-scaled drug dosing in patients with a high body mass index (Nyman 2016), where an underestimate of lean mass would under-dose. Most calculators print the James number regardless. This one detects when your inputs sit past the peak and flags it — if you see that warning, use Boer or Hume, or measure body fat directly.
When this calculator is wrong
The same limit that breaks BMI breaks all three formulas here, and it's whole categories, not a rounding error: none of them can tell muscle from fat, because none takes body composition as an input. Run a 200 lb (90.7 kg), 6'0" (183 cm) man at 12% body fat through BMI and it returns 27.1 — "overweight" by WHO (25.0–29.9) — while that 12% body fat sits squarely in the athlete range (6–13%, ACE). The lean-mass formulas make the same mistake: at a given height and weight they hand back a single lean-mass number whether you're that lean lifter or someone carrying far more fat, because the input that would separate the two is exactly the one they don't have. The muscular lifter's true lean mass is well above the estimate; the formula can't know it.
The counter-case is the general, non-athletic adult, which is who these equations were built to describe. If you carry roughly average muscle for your size, the three-formula range brackets your lean mass well enough to be a useful reference — a first pass, not a measurement. The error bites at the edges: the heavily muscled read low, and people who've lost muscle can hit a normal-looking estimate while carrying too much fat for their frame. When the formula and a body-fat measurement disagree, trust the measurement.
What to do with the result
Read the output as a range, and if you have a body-fat percentage, let the direct number override it. Lean body mass is most useful as an input to other math: protein targets are often set per pound of lean mass rather than total weight, and the Katch-McArdle BMR formula — the most accurate resting-metabolism estimate for lean and muscular people — takes lean body mass directly. Feed the direct-method figure into those, not one of the three estimates, whenever you have a real body-fat reading.
If you're tracking change over time, hold the method constant and watch the trend rather than the absolute number. Pick one formula (or, better, one body-fat method) and re-run it every few weeks; a rising lean-mass figure at a stable or falling body weight is the signal that a training block is adding muscle while holding fat. Switching formulas between measurements just adds the 10-plus-pound spread back into your data.
Common questions
- How do I calculate my lean body mass?
- If you know your body-fat percentage, lean body mass = weight × (1 − body fat/100): a 180 lb man at 18% is 180 × 0.82 = 147.6 lb. Without a body-fat number, an equation estimates it from height, weight, and sex — Boer for men is 0.407 × weight(kg) + 0.267 × height(cm) − 19.2. This calculator runs Boer, James, and Hume together and shows the range, because no single one is definitive.
- Which lean body mass formula is most accurate?
- None is "accurate" in the sense of measuring you — all three are height-and-weight regressions with no body-composition input. Boer (1984) is the most commonly recommended default and holds up reasonably across normal-weight adults; James (1976) becomes unreliable at a high body mass index because it runs backwards (see above). The genuinely accurate move is to measure body fat and use the direct identity instead.
- Is lean body mass the same as fat-free mass?
- Almost, and these formulas treat them as one. Strictly, "lean body mass" includes a small amount of essential lipid in the nervous system and organs (roughly 2–3% of body weight), while "fat-free mass" excludes all lipid. The difference is smaller than the disagreement between the three formulas, so in practice the terms are used interchangeably and these equations don't distinguish them.
- Is lean body mass the same as muscle mass?
- No. Lean body mass includes bone, organs, connective tissue, and body water on top of muscle. Skeletal muscle is only about a third to a little under half of lean body mass in most adults, so your muscle mass is well below your lean-mass figure. A calculator that reports "muscle mass" from height and weight is estimating a fraction of an estimate.
- Why do the three formulas give different numbers?
- They were derived by different authors from different reference groups, measured by different methods, in different decades. For a 5'10" 180 lb man they run from 126.9 lb (Hume) to 138.5 lb (James) — about 11.6 lb (5.2 kg) apart. The spread is the point: it shows how much a height-and-weight estimate depends on which equation you happen to use.
- Can I calculate lean body mass without knowing my body fat?
- Yes — that's what Boer, James, and Hume are for, and this page shows all three. But they're estimates that assume an average body composition for your size. If you have a body-fat reading from a tape test, calipers, or a DEXA scan, enter it: the direct calculation is closer to the truth and is the number the Katch-McArdle metabolism formula wants.