CalcVitalsLean Body Mass Calculator
Lean body mass is your total weight minus fat mass. The Boer formula estimates it from weight and height: a man of 70 kg and 175 cm has a lean body mass of about 56.0 kg — roughly 80% of his total weight.
How it works
Lean body mass is everything in you that is not fat: muscle, bone, organs, blood and water. It is the metabolically active part of you — the tissue that burns energy at rest, moves you around, and determines how much protein you need. Dieters care about it because the goal is never really to lose weight; it is to lose fat while keeping this number as intact as possible.
The Boer formula (1984) estimates lean mass from weight, height and sex, and holds up well against direct measurement for most adults. The James formula, shown alongside, approaches from a different fit; where the two agree closely you can be reasonably confident, and where they drift apart — usually at very high or low weights — treat both as rough.
A practical use: protein guidance for preserving muscle is usually quoted per kilogram of lean mass rather than per kilogram of total weight, which matters a great deal when total weight includes a lot of fat. The protein calculator on this site works from total weight with goal-based factors; dividing its answer by your lean mass here is a good sanity check.
The method
Boer (1984): men LBM = 0.407·weight + 0.267·height − 19.2; women LBM = 0.252·weight + 0.473·height − 48.3 (kg, cm). James: men 1.1·W − 128·(W/H)²; women 1.07·W − 148·(W/H)². Implied body fat = (1 − LBM ÷ weight) × 100.
Frequently asked questions
What is a good lean body mass?
There is no universal target — it scales with height and frame. What matters is the direction: lean mass stable or rising while fat falls is the signature of a diet and training plan that is working.
Is lean body mass the same as muscle mass?
No. Muscle is only part of it — lean mass also counts bone, organs, skin and body water. Skeletal muscle is roughly half of lean mass in a typical adult.
Why protect lean mass while dieting?
Lean tissue drives resting metabolism and strength. Losing it makes the smaller body you end up with burn less and perform worse, which is one reason crash diets rebound so reliably.