Lean Body Mass Calculator
Estimate lean body mass and fat mass using the Boer, James and Hume equations, with the fat-free mass index.
How to use this calculator
- 1Enter your height and weight.
- 2Compare the three estimates. A wide spread means the prediction is less reliable for your build.
- 3If you know your actual body fat percentage, lean mass = weight × (1 − body fat), which beats any of these estimates.
How the calculation works
Boer (male): LBM = 0.407w + 0.267h − 19.2
Boer (female): LBM = 0.252w + 0.473h − 48.3- w
- Body weight in kilograms
- h
- Height in centimetres
Three published equations are shown because they disagree, particularly at higher body weights, and the spread is useful information.
The James equation includes a squared weight-to-height term, so it behaves differently — and less reliably — at the extremes.
FFMI normalisation to 1.8 m allows comparison between people of different heights, which raw FFMI does not.
Worked example
A male of 175 cm and 75 kg
- 1.Boer: 0.407 × 75 + 0.267 × 175 − 19.2 = 30.5 + 46.7 − 19.2 = 58.0 kg.
- 2.James: 1.1 × 75 − 128 × (75/175)² = 82.5 − 23.5 = 59.0 kg.
- 3.Hume: 0.3281 × 75 + 0.33929 × 175 − 29.5336 = 24.6 + 59.4 − 29.5 = 54.5 kg.
- 4.The average is about 57.2 kg, implying 17.8 kg of fat mass — around 23.7% body fat.
Result: About 57 kg lean mass
What lean body mass actually includes
Lean body mass is everything in the body that is not fat: skeletal muscle, bone, organs, blood, connective tissue and body water. It is a broader category than “muscle mass” — muscle is roughly half of it — which is a common point of confusion, since the two terms are often used loosely as if interchangeable.
Why it gets measured separately from total weight
Lean tissue is metabolically active in a way fat tissue largely is not: it drives the majority of resting energy expenditure, which is exactly why the Katch–McArdle BMR formula works from lean mass rather than total weight. The same logic shows up in drug dosing, where doses for many medications are calculated against lean rather than total body mass, and in strength and physique sports, where tracking lean mass separately from scale weight reveals whether a training block actually built muscle or simply changed the number on the scale.
Three equations, three different eras
The equations compared here were each fitted to a different reference population using the measurement techniques available at the time.
- Hume (1966) — one of the earliest widely used prediction equations, derived from a relatively small clinical sample.
- James (1976) — incorporates a squared weight-to-height term, which makes it behave differently — and less reliably — at the extremes of body size compared with the other two.
- Boer (1984) — generally regarded as the most accurate of the three against modern reference methods, and the one most often cited in current clinical literature.
The fat-free mass index, and its natural ceiling
Fat-free mass index (FFMI) applies the same logic as BMI — dividing a mass by height squared — but to lean mass rather than total weight, and is normalised to a standard height of 1.8 metres so people of different heights can be compared fairly. Typical untrained values sit around 19–20 for men and 15–16 for women; well-trained natural athletes commonly reach the low-to-mid 20s. Values much beyond that are difficult to reach through training and diet alone, which is part of why FFMI occasionally comes up in discussions of natural versus assisted muscular development — though it remains only a population-level statistical observation, not a test capable of proving anything about any one individual.
What this assumes, and where it stops
Assumptions
- The equations apply to adults with typical body composition for their height and weight.
Limitations
- Predicted from height and weight only, so they cannot tell a muscular person from a heavier sedentary one.
- Accuracy falls at the extremes — very lean, very muscular, or a BMI above roughly 35.
- DEXA, hydrostatic weighing and air displacement plethysmography all measure this directly and far more accurately.
Common questions
Is lean body mass the same as muscle mass?
No. Lean body mass includes muscle, bone, organs, blood and water — everything except fat. Skeletal muscle is roughly half of it. So a 57 kg lean mass corresponds to something like 28 kg of actual muscle.
What is a good FFMI?
Around 19–20 is typical for an untrained male and 15–16 for an untrained female. Well-trained natural athletes commonly reach 22–24. Sustained values above about 25 in males are very rarely achieved without pharmacological help.
Sources
- Boer P — Estimated lean body mass as an index for normalization — American Journal of Physiology (1984)
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
Related calculators
Tools people commonly use alongside the lean body mass calculator.