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

The formula, worked through

• Formula Lean Body Mass Formula (Boer)

  Lean body mass formula (Boer 1984). Male: LBM = 0.407·mass_kg + 0.267·height_cm − 19.2. Female: LBM = 0.252·mass_kg + 0.473·height_cm − 48.3.

• Formula Body Fat Percentage Formula (US Navy)

  US Navy circumference-based body fat. Men: 86.010·log10(waist−neck) − 70.041·log10(height) + 36.76. Women: 163.205·log10(waist+hip−neck) − 97.684·log10(height) − 78.387.

• Formula FFMI Formula: Kouri-Adjusted Fat-Free Mass Index

  FFMI formula: FFMI = LBM_kg / (height_m)² + 6.1·(1.8 − height_m). The +6.1·(1.8−h) normalizes to a 1.8 m reference. Natural ceiling ~25 male, ~22 female.

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Background on the Lean Body Mass Calculator

• Guide How to Measure Body Fat at Home: 4 Methods Compared

  How to measure body fat at home: the Navy tape method, a smart scale, skinfold calipers or a visual estimate, the error each carries and which to repeat.

• Examples Lean Body Mass Examples

  Explore diverse lean body mass examples across fitness, health, and wellness scenarios. Understand its critical role in tracking real progress beyond just.

• Checklist Body Recomposition Checklist

  Plan body recomposition with this actionable checklist. Simultaneously build muscle and burn fat through precise nutrition, effective training.

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

  Evidence-based body composition statistics: average body fat percentages, lean mass norms by sex and age, measurement accuracy, and natural FFMI ceilings.

• Learn What Is Lean Body Mass? Simply Explained

  Understand Lean Body Mass: its definition, why it matters for health and fitness, how to calculate it, and its impact on metabolism and body composition goals.

• Examples Body Fat Examples

  Explore diverse real-world body fat examples, from sedentary professionals to athletes and new mothers, revealing unique insights into health, performance.

Articles

• 10 min read Body Composition: The Four Lenses

  Body composition through four lenses: BMI, FFMI, measured body fat, and energy balance. What each metric answers, where it fails, and the deep dives.

• 11 min read Bod Pod vs DEXA vs Calipers: Body Fat Accuracy

  Bod Pod runs about +/-2-3% body-fat error, DEXA +/-1-2%, calipers +/-3-4%. Compare accuracy, cost, and cadence to pick a body composition method for athletes.

• 12 min read Maximum Natural FFMI: The Confounders That Bend the Limit

  Maximum natural FFMI is a soft limit, not a wall: Kouri 1995 put naturals near 25, but glycogen, height over 195 cm, and contest-day bias bend the number.

By goal

• Build strength and muscle
• Benchmark your strength
• Understand body composition

Use it from code

The same calculation as a JavaScript module you can import. It runs where you import it, with no request, key or rate limit.

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

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