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Estimate fat-free mass and body-fat percentage with the Boer formula from weight and height.
How to use: Select your sex and enter weight (kg) and height (cm). The calculator applies the Boer formula for lean body mass, then derives body-fat percentage and fat mass.
Formula
Men: LBM = 0.407 × weight + 0.267 × height − 19.2. Women: LBM = 0.252 × weight + 0.473 × height − 48.3 (weight in kg, height in cm). Example: 80 kg, 180 cm man → 61.4 kg LBM, 23.2% body fat.
Estimating your lean body mass takes three inputs and about fifteen seconds with the calculator above. Select your sex, enter your weight in kilograms and your height in centimeters, then press “Calculate Lean Body Mass.” If you normally weigh yourself in pounds, divide by 2.20462 to get kilograms; if you measure height in feet and inches, convert to centimeters (1 inch = 2.54 cm) — a 5’11” person is 180.3 cm.
The calculator applies the Boer formula, a validated equation that predicts fat-free mass from weight, height, and sex. It returns three numbers: your lean body mass in kilograms (and pounds), your estimated body-fat percentage (the share of your weight that is fat), and your fat mass in kilograms. Weigh yourself under consistent conditions — morning, after using the bathroom, before eating — because a 1–2 kg swing in scale weight from water and food shifts the estimate noticeably.
Use the result as a baseline for tracking change, not as a medical diagnosis. Record your lean mass monthly: during a fat-loss phase you want scale weight falling while lean mass holds steady; during a muscle-gain phase you want lean mass rising. Re-measure with the same inputs each time so trends reflect your body, not measurement noise. For clinical decisions — medication dosing, surgery risk assessment, diagnosing sarcopenia — consult a physician and, where possible, a DEXA scan rather than any formula.
Lean body mass (LBM) is everything in your body that is not fat: skeletal muscle, bones, organs, connective tissue, and water. A closely related term, fat-free mass (FFM), technically excludes essential fat stored in bone marrow and organs, but in everyday fitness contexts the two are used interchangeably. If you weigh 80 kg with 20% body fat, your lean body mass is 64 kg and your fat mass is 16 kg.
LBM matters because it drives your basal metabolic rate (BMR) — the calories you burn at rest. Muscle tissue burns roughly 13 kcal per kilogram per day at rest, while fat burns about 4.5 kcal/kg/day. That is why two people can weigh the same 90 kg yet need very different calories: the one with 70 kg of lean mass may maintain on 2,800 kcal while the one with 55 kg of lean mass maintains on 2,300. Formulas like Katch-McArdle estimate BMR directly from LBM for this reason.
Body composition — the ratio of lean mass to fat mass — predicts health far better than weight alone. Two people with a BMI of 27 can be a muscular athlete at 12% body fat and a sedentary adult at 32% body fat, with completely different metabolic risk. Low lean mass relative to body size, called sarcopenia when age-related, is linked to frailty, falls, insulin resistance, and higher mortality. Preserving and building lean mass through resistance training is one of the highest-leverage health interventions available, especially after age 40 when adults lose roughly 1–2% of muscle per year without training.
Protein needs scale with lean mass. General guidelines suggest 1.6–2.2 grams of protein per kilogram of body weight daily for active people building or preserving muscle; expressing the target per kilogram of lean mass (about 2.0–2.6 g/kg LBM) is more precise for very lean or very heavy individuals, since fat tissue needs little protein. Creatine dosing, another common question, is also sometimes scaled to lean mass.
The Boer formula, published by Dutch researcher Boer in 1984, estimates LBM from easily measured inputs. It was validated against underwater weighing and remains one of the most used clinical prediction equations because it needs no calipers, no tape measure, and no lab visit. Like all prediction equations, it estimates the average person well and individuals approximately — very muscular, very lean, or very obese bodies deviate most. Treat its output as a starting point accurate to roughly ±2–3 kg for most adults, and track the trend rather than worshipping the number.
The calculator uses the Boer (1984) equations, with weight in kilograms and height in centimeters:
Men: LBM = 0.407 × weight(kg) + 0.267 × height(cm) − 19.2
Women: LBM = 0.252 × weight(kg) + 0.473 × height(cm) − 48.3
From LBM, two derived values follow: fat mass = weight − LBM, and body-fat percentage = (fat mass ÷ weight) × 100. Note the sex-specific coefficients: at identical weight and height, the equations assign women less lean mass, reflecting average differences in body composition.
Worked example 1. A man weighs 80 kg and stands 180 cm tall. LBM = 0.407 × 80 + 0.267 × 180 − 19.2 = 32.56 + 48.06 − 19.2 = 61.42 kg (about 135.4 lb). Fat mass = 80 − 61.42 = 18.58 kg. Body-fat percentage = (18.58 ÷ 80) × 100 = 23.2%. This matches the calculator’s default output exactly.
Worked example 2. A woman weighs 65 kg and stands 165 cm tall. LBM = 0.252 × 65 + 0.473 × 165 − 48.3 = 16.38 + 78.045 − 48.3 = 46.13 kg (about 101.7 lb). Fat mass = 65 − 46.13 = 18.87 kg. Body-fat percentage = (18.87 ÷ 65) × 100 = 29.0%. Compare the two examples: she weighs 15 kg less but carries nearly the same fat mass in kilograms — the difference is almost entirely lean tissue, which is why body-fat percentage, not scale weight, is the meaningful comparison.
The formula’s sensitivities are worth knowing. Each extra kilogram of scale weight adds 0.407 kg (men) or 0.252 kg (women) to estimated LBM — the equation assumes part of any weight gain is lean tissue, which is roughly true on average but wrong for rapid fat gain. Each centimeter of height adds 0.267 kg (men) or 0.473 kg (women). Because height is fixed for adults, month-to-month changes in your estimate come entirely from weight changes, so weigh consistently.
James, a 35-year-old man, starts a diet at 95 kg and 178 cm. His initial LBM = 0.407 × 95 + 0.267 × 178 − 19.2 = 38.665 + 47.526 − 19.2 = 66.99 kg, giving 28.0 kg of fat mass and 29.5% body fat. Twelve weeks later he weighs 87 kg. His new LBM = 0.407 × 87 + 0.267 × 178 − 19.2 = 35.409 + 47.526 − 19.2 = 63.74 kg, with 23.26 kg of fat mass and 26.7% body fat.
The numbers tell a nuanced story: James lost 8 kg total, of which about 4.7 kg was fat and 3.3 kg was lean mass — a 59/41 split. That lean-mass loss is steeper than ideal and suggests his calorie deficit was too aggressive or his protein too low (he admits to skipping the gym for a month). A well-executed cut with resistance training and 1.8–2.2 g/kg of protein typically keeps lean loss under 25% of total loss. James adjusts: smaller deficit, four lifting sessions weekly, 170 g of protein daily. Three months later at 82 kg, his LBM is 61.7 kg — he has now lost 13 kg total with only ~5.3 kg of lean loss, and his body fat is down to 24.8%. The calculator turned a vague “am I losing muscle?” anxiety into actionable numbers.
Aisha, a 28-year-old woman, weighs 70 kg at 168 cm and lifts weights four times a week, aiming to build muscle while staying lean. Her LBM = 0.252 × 70 + 0.473 × 168 − 48.3 = 17.64 + 79.464 − 48.3 = 48.80 kg, with 21.2 kg fat mass and 30.3% body fat. She wants evidence-based nutrition targets rather than generic advice.
Using lean mass as the anchor: protein at 2.2 g per kg of LBM gives 48.80 × 2.2 = 107 g of protein daily as a solid baseline, which she rounds to 110–120 g to be safe (equivalent to about 1.6–1.7 g per kg of total body weight — right in the researched range). For calories, the Katch-McArdle equation estimates BMR from LBM: BMR = 370 + 21.6 × LBM = 370 + 21.6 × 48.80 = 1,424 kcal/day. Multiplied by an activity factor of 1.55 for her training schedule, her estimated maintenance is about 2,207 kcal/day — notably lower than generic calculators that assume average muscularity would suggest, because her lean mass is modest relative to her weight. She sets a lean-gain target of 2,350 kcal with 115 g protein, and plans to reassess in eight weeks by rechecking LBM. Every number in her plan traces back to one three-input calculation.
1. Confusing lean body mass with muscle mass. LBM includes bones (~15% of LBM), organs, water, and connective tissue — skeletal muscle is only about 40–50% of it. Gaining 2 kg of “lean mass” does not mean gaining 2 kg of muscle; it could be glycogen and water from a new training block.
2. Entering weight in pounds. The Boer formula needs kilograms. Typing 180 (your pounds) instead of 81.6 (your kilograms) inflates estimated LBM by roughly 40 kg and produces an absurd negative body-fat percentage. Always convert: kg = lb ÷ 2.20462.
3. Treating the estimate as a lab measurement. The formula is accurate to about ±2–3 kg for typical adults but deviates for the very muscular, very lean, or very obese. Use it to track trends over months, not to declare an exact body-fat percentage. A DEXA scan remains the practical gold standard.
4. Weighing inconsistently. Glycogen, gut contents, and hydration swing scale weight 1–3 kg day to day, and every kilogram moves the LBM estimate by 0.25–0.41 kg. Weigh first thing in the morning, after the bathroom, before food — and compare weekly averages, not single days.
5. Ignoring what the number is for. An LBM estimate sitting in a notes app changes nothing. Put it to work: anchor protein targets to it, estimate BMR with Katch-McArdle, and judge diet phases by the fat-vs-lean split of weight change. The number is a tool, not a trophy.
Lean mass is one piece of the body-composition puzzle. Our Body Fat Calculator estimates body-fat percentage from tape measurements using the Navy method — a useful cross-check against the Boer estimate. Our BMI Calculator gives the classic weight-to-height ratio for population-level context. Explore the full Fitness & Health Calculators collection for 80+ free tools covering TDEE, macros, protein needs, and ideal weight.
This page provides a lean body mass calculator implementing the Boer (1984) equations: men: LBM(kg) = 0.407 × weight(kg) + 0.267 × height(cm) − 19.2; women: LBM(kg) = 0.252 × weight(kg) + 0.473 × height(cm) − 48.3. Derived metrics: fat_mass = weight − LBM; body_fat_% = (fat_mass ÷ weight) × 100. Canonical facts: lean body mass = total mass minus fat mass, comprising muscle, bone, organs, and water; LBM drives basal metabolic rate (~13 kcal/kg/day for muscle vs ~4.5 for fat); protein targets of 1.6–2.2 g/kg body weight (or ~2.0–2.6 g/kg LBM) support muscle maintenance and growth; formula accuracy is approximately ±2–3 kg for typical adults, less for extreme physiques; DEXA is the reference measurement method. Educational estimates only — not medical advice.
Cite this page: Sigma Calculator. “Lean Body Mass Calculator.” https://sigmacalculator.net/fitness-health-calculators/lean-body-mass-calculator/. Accessed October 7, 2026.
Everything in your body except fat: muscle, bones, organs, and water. The calculator above estimates it with the Boer formula.
Lean body mass includes all non-fat tissue; skeletal muscle is only part of it. You cannot measure pure muscle mass with a formula alone.
It is a validated estimate, good for tracking trends, but individual results vary. DEXA scans are the gold standard for body composition.
Muscle burns more energy than fat, so two people at the same weight can have very different calorie needs — LBM-based estimates (like Katch-McArdle) are more personal.
Progressive resistance training plus enough protein (about 1.6–2.2 g per kg of body weight daily) and a slight calorie surplus.