Lean Body Mass Calculator

How to use

1. 1Pick a preset (adult male, adult female, lean athlete, older adult, or child) to load realistic inputs, or fill in your own values.
2. 2Choose your unit system: Metric uses kilograms and centimeters, Imperial uses pounds, feet, and inches.
3. 3Select sex. Each adult formula has different coefficients for male and female bodies; the Peters pediatric formula does not depend on sex.
4. 4Enter weight and height. Optionally enter age in years; children under 13 switch the primary formula to the Peters pediatric equation.
5. 5Read the primary lean body mass result on the right, along with body fat mass and body fat percent estimated from total weight minus LBM.
6. 6Scroll to the comparison table to see all four formulas side by side, then use Copy summary to save or share the full result.

About this tool

Lean Body Mass Calculator estimates lean body mass (LBM), body fat mass, and body fat percent directly from your sex, weight, and height using four well-known closed-form equations: Boer (1984), James (1976), Hume (1966), and the pediatric Peters (2011) formula. LBM is the part of body weight that is not stored fat: muscle, bone, organs, skin, blood, and the small amount of essential structural fat in cell membranes. It is the dosing reference for many anesthetic, oncology, and weight-based medications because dosing on total body weight overshoots in patients with high body fat, and a number of energy-expenditure equations (Katch-McArdle, Cunningham) take LBM as their primary input rather than total weight. Unlike the Body Fat Calculator, which uses the U.S. Navy method with neck, waist, and hip circumferences, this tool needs only height and weight, so it is the right starting point when you do not have a tape measure or when you want a quick adult or pediatric LBM estimate. Each equation has separate male and female coefficients (the Peters formula is sex-independent and intended for children up to about 13 years old). The calculator runs every applicable formula in parallel, highlights the primary equation based on age group (Boer for adults, Peters for children), shows the spread across formulas in a side-by-side table, and reports a simple average across the formulas that apply to the age group as a single number you can quote. The result panel also derives body fat mass (total weight minus LBM) and body fat percent (fat mass divided by total weight), letting the same tool answer both lean and fat questions from one set of inputs. Metric uses kilograms and centimeters; imperial uses pounds, feet, and inches with automatic conversion. Quick presets cover an adult male, adult female, a lean athlete, an older adult, and a child sample so you can sanity check the math against realistic numbers before entering your own. Useful for personal body composition tracking when you do not want to take circumferences, clinicians sanity checking a weight-based dose against LBM, nutrition coaches building a starting calorie target on the Katch-McArdle BMR equation, and researchers comparing LBM equations on the same subject. All math runs locally in your browser; weight, height, and age never leave your device. Not medical advice; if you are calculating a medication dose, follow your local clinical guideline.

Free to use. Works in your browser. No signup, no login.