Lean Body Mass Calculator

What does the Lean Body Mass Calculator calculate, and which formula does it use?

The Lean Body Mass Calculator estimates fat-free mass from height, weight, and sex using three published formulas — Boer (1984), James (1976), and Hume (1966) — without any body-fat measurement. Boer for men, for example, is 0.407 × weight (kg) + 0.267 × height (cm) − 19.2. It averages the three estimates and reports the body-fat percentage each one implies.

Last updated March 3, 2026.

How to use it

1. Pick Metric or US units, then enter sex, weight and height. The Boer, James and Hume formulas estimate lean body mass from those alone; the headline is their average.
2. LBM = total weight minus fat mass: muscle, bone, organs and water. These formulas assume a typical build, so if you lift, a measured body fat gives a truer number: a 185 lb person at 20% body fat has 148 lb of lean mass.
3. For a fat-loss phase, a protein target of 1.0 to 1.4 g per lb of lean mass (2.3 to 3.1 g per kg, Helms et al. 2014) suits lean, trained lifters; base it on lean mass rather than total weight if you carry a lot of fat.
4. During a cut, aim for most of the weight lost to be fat: if you lose 10 lb and at least 7.5 lb of it is fat, the deficit and protein are working. A larger share of lean mass lost signals too steep a deficit or too little protein.
5. Re-run monthly and compare LBM across months: rising LBM while weight holds or drops = muscle gain or successful recomposition. Stable LBM while body fat drops = textbook fat loss.

Questions people usually ask

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Background on the Lean Body Mass Calculator

• Statistics Body Composition Statistics: Body Fat & Lean Mass Norms

  Body composition statistics: body fat at each BMI by sex and age, muscle mass by sex, how accurate each measurement method is, and the FFMI 25 line.

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• 7 min read BMI vs FFMI vs Body Fat: A 178cm/80kg Male Through Three Lenses

  Run BMI, FFMI, and Navy body-fat on the same 178cm/80kg male. Where the BMI overweight flag clashes with FFMI 22, and what to actually believe.

• 7 min read FFMI for the Intermediate Female Lifter: A 60kg/165cm/22% Walkthrough

  FFMI for a 60kg, 165cm, 22% body-fat female intermediate lifter: adjusted 18.14 walkthrough, percentile context, and why the Kouri 25 cap was male-only.

By goal

• Body Composition

Use it from an agent or code

Any agent can run this calculator by URL: put the inputs in the query string and get JSON back, with the inputs that used defaults and a line to cite (add format=md for markdown). MCP clients reach it through https://aifithub.io/mcp.

https://aifithub.io/api/run/lean-body-mass-calculator?sex=male&weight_kg=80&height_cm=178

In code, import the same module; it runs where you import it, with no request or key.

import { compute } from "https://aifithub.io/engines/lean-body-mass-calculator.js";

Input and output contract, TypeScript types and the guide for agents.