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Lean Body Mass Calculator

Lean mass and fat mass from measurements or body fat.

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Formula estimates

Three published equations, which routinely differ by a kilogram or two.

FormulaLean massImplied body fat

Lean body mass

—

Composition

Fat mass—
Lean as % of weight—
Fat-free mass index—
Katch-McArdle BMR—

—

Protein at 2 g per kg of lean mass

A known body fat percentage gives an exact split; without one, the formulas are population estimates fitted to different samples, which is why they disagree. Fat-free mass index typically runs about 16 to 20 for women and 18 to 22 for men. Not medical advice.

Quick Tool Overview

The Lean Body Mass Calculator splits your weight into lean mass and fat mass. With a known body fat percentage the split is exact; without one, the Boer, James and Hume formulas estimate lean mass from height, weight and sex, and are shown together.

What it does
Splits body weight into lean mass and fat mass
You give it
Height, weight, sex and optionally body fat percentage
You get back
Lean mass, fat mass, FFMI and a Katch-McArdle BMR
Best for
Protein targets, tracking recomposition and metabolic estimates
Calculators Free, unlimited use No sign-up Runs in your browser

About the Lean Body Mass Calculator

Split your body weight into lean mass and fat mass. If you know your body fat percentage the arithmetic is exact; if you do not, three published formulas — Boer, James and Hume — estimate lean mass from height, weight and sex.

Lean body mass is the more useful number in several places where total weight is normally used. Drug dosing, protein targets and the Katch-McArdle metabolic rate formula all key off lean mass rather than scale weight, because fat tissue is metabolically quiet by comparison. Tracking it also tells you what a weight change actually was: losing four kilos of which three were lean is a different outcome from losing four kilos of fat, and the scale alone cannot tell the difference.

Key features of the Lean Body Mass Calculator

Three estimation formulas

Boer, James and Hume are calculated together, since they routinely differ by a kilogram or two.

Exact split when you know body fat

A measured percentage skips the estimates entirely and gives the arithmetic answer.

Fat-free mass index

FFMI normalises lean mass for height, which makes it a fairer comparison than raw kilograms.

Feeds Katch-McArdle

Lean mass drives the most accurate BMR formula, and the result is shown so you do not need a second tool.

Explains why it matters

Losing four kilos of which three were lean is a different outcome from losing four kilos of fat.

How to use the Lean Body Mass Calculator

  1. 1

    Enter height and weight

    In metric or imperial units.

  2. 2

    Add body fat if you have it

    A known percentage gives an exact split. Without it, three formulas estimate lean mass instead.

  3. 3

    Compare the estimates

    Boer, James and Hume are calculated together, since they routinely differ by a kilogram or two.

  4. 4

    Read the derived figures

    Fat mass, fat-free mass index and the Katch-McArdle metabolic rate that follows from lean mass.

Frequently asked questions

Everything in your body that is not fat: muscle, bone, organs, connective tissue and body water. It is sometimes used interchangeably with fat-free mass, though strictly fat-free mass excludes essential fat too.

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