What is lean body mass?

Lean body mass (LBM) is the total weight of your body minus fat mass. It includes muscles, bones, organs, water, and connective tissue. LBM is important for calculating protein needs, BMR (Katch-McArdle formula), and setting realistic fitness goals.

How is lean body mass calculated?

The simplest method: LBM = Total Weight × (1 − Body Fat%). The Boer formula (male: 0.407W + 0.267H − 19.2; female: 0.252W + 0.473H − 48.3) uses height and weight without needing body fat %. All methods are estimates.

What is FFMI (Fat-Free Mass Index)?

FFMI = Lean Body Mass (kg) / Height (m)². It is the muscle-mass equivalent of BMI. An FFMI of 20–22 is above average; 23–25 is excellent; above 25 may indicate anabolic steroid use in natural athletes. Average men score around 18–19.

Why is lean body mass important?

Lean body mass drives your BMR — more muscle means more calories burned at rest. It is also used for protein targets (1g per lb of LBM is a common recommendation), drug dosing, and tracking body composition changes more accurately than scale weight alone.

Lean Body Mass Calculator

Calculate lean body mass (LBM) from body fat percentage or using Boer, James, and Hume formulas. Includes FFMI, Katch-McArdle BMR, skeletal muscle estimate, and protein intake recommendation.

Weight ?

lbs

Body Fat % ?

Lean Body Mass

—

Fat Mass —

LBM Ratio —

Extended More scenarios, charts & detailed breakdown ▾

Weight

lbs

Body Fat %

Lean Body Mass

—

Fat Mass —

LBM (kg) —

Professional Full parameters & maximum detail ▾

Weight

lbs

Height

inches

Body Fat %

Sex

Age

years

LBM by Formula

LBM (from Body Fat %) —

LBM — Boer Formula —

LBM — James Formula —

LBM — Hume Formula —

Body Composition

Fat Mass —

FFMI (Fat-Free Mass Index) —

FFMI Category —

Performance Metrics

BMR (Katch-McArdle) —

Recommended Protein —

How to Use This Calculator

Enter your weight and body fat percentage for the direct LBM calculation.
Use the Boer Formula tab to estimate LBM from height, weight, and sex — no body fat % needed.
Use the Goal Setting tab to find your target weight at a lower body fat % while preserving lean mass.
Use the Professional tab for a comparison of Boer, James, and Hume formulas plus FFMI and Katch-McArdle BMR.

Formula

LBM = Weight × (1 − Body Fat%)
Boer (male): LBM = 0.407W(kg) + 0.267H(cm) − 19.2
FFMI = LBM(kg) / Height(m)²
Katch-McArdle BMR = 370 + 21.6 × LBM(kg)

Example

Example: 170 lb person with 18% body fat: Fat mass = 30.6 lbs | LBM = 139.4 lbs (63.3 kg). FFMI = 63.3 / (1.78)² = 20.0 — above average.

Frequently Asked Questions

Lean body mass (LBM) is the total weight of your body minus fat mass. It includes muscles, bones, organs, water, and connective tissue. LBM is important for calculating protein needs, BMR (Katch-McArdle formula), and setting realistic fitness goals.
The simplest method: LBM = Total Weight × (1 − Body Fat%). The Boer formula (male: 0.407W + 0.267H − 19.2; female: 0.252W + 0.473H − 48.3) uses height and weight without needing body fat %. All methods are estimates.
FFMI = Lean Body Mass (kg) / Height (m)². It is the muscle-mass equivalent of BMI. An FFMI of 20–22 is above average; 23–25 is excellent; above 25 may indicate anabolic steroid use in natural athletes. Average men score around 18–19.
Lean body mass drives your BMR — more muscle means more calories burned at rest. It is also used for protein targets (1g per lb of LBM is a common recommendation), drug dosing, and tracking body composition changes more accurately than scale weight alone.

Related Calculators

Sources & References (5) ▾

Boer P — Estimated lean body mass as an index for normalization of body fluid volumes in man. Am J Physiol. 1984;247(4):F632-636 — American Journal of Physiology
Katch VL et al. — Prediction of body density from simple anthropometric measurements in college-aged men and women. Hum Biol. 1973;45(3):445-455 — Human Biology
Cunningham JJ — Body composition as a determinant of energy expenditure: a synthetic review. Am J Clin Nutr. 1991;54(6):963-969 — American Journal of Clinical Nutrition
Kouri EM et al. — Fat-free mass index in users and nonusers of anabolic-androgenic steroids. Clin J Sport Med. 1995;5(4):223-228 — Clinical Journal of Sport Medicine
American College of Sports Medicine — ACSM's Guidelines for Exercise Testing and Prescription, 11th ed. (2022) — ACSM