Free lean body mass calculator
Find your lean body mass in two seconds. Enter your weight, height, and sex in metric or imperial. The calculator returns your lean (fat-free) body mass in kilograms using the Boer formula, your fat mass, and your lean-mass percentage — updated live, as you type.
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Formula-based estimates have ±2–4% error. DEXA or underwater weighing provides more accurate measurements.
Results are estimates. Consult a professional.
What is lean body mass?
Lean body mass (LBM) is the weight of everything in your body that is not fat — your muscle, bone, organs, skin, and the water held inside them. It is simply your total body weight minus your fat mass. Where the scale tells you how heavy you are overall, lean body mass tells you how much of that weight is working tissue rather than stored fat. This lean body mass calculator estimates that figure in kilograms from just your weight, height, and sex, using the Boer formula, the moment you enter your numbers.
LBM matters because it is the part of your body that drives metabolism, strength, and how your body handles medication. Two people can weigh exactly the same and have very different lean body masses — one carrying more muscle, the other more fat. That difference is invisible on the scale but central to fitness, nutrition, and clinical dosing, which is why doctors, dietitians, and coaches track lean mass alongside total weight.
The Boer formula for lean body mass
This calculator uses the Boer formula, published by P. Boer in 1984 and one of the most widely cited predictive equations for lean body mass. Rather than scanning your body, it estimates LBM statistically from the relationship between weight, height, and sex observed in healthy adults. There are two versions — one for men and one for women — because men carry a higher share of lean mass on average.
- Weight (kg) — your total body weight. Heavier people carry more lean tissue, so weight is the largest driver in the men's equation.
- Height (cm) — taller frames support more muscle and bone, which is why height carries more weight in the women's equation.
- Sex — selects which of the two equations runs. The formulas are not interchangeable; using the wrong one shifts the estimate by several kilograms.
How to calculate your lean body mass
You only need three things: your weight, your height, and your sex. The calculator handles unit conversion, but the Boer formula itself runs on kilograms and centimetres, so the steps below use metric inputs.
- Pick the right equation. Men use LBM = 0.407 × kg + 0.267 × cm − 19.2; women use LBM = 0.252 × kg + 0.473 × cm − 48.3.
- Multiply your weight by its coefficient. For a man at 80 kg: 0.407 × 80 = 32.56.
- Multiply your height by its coefficient. At 180 cm: 0.267 × 180 = 48.06.
- Add the two and subtract the constant. 32.56 + 48.06 − 19.2 = 61.42 kg of lean body mass.
- Subtract LBM from weight to get fat mass. 80 − 61.42 = 18.58 kg, which is about 23% body fat.
A worked example using the lean body mass calculator
David is 180 cm tall and weighs 80 kg. He selects male and enters the two numbers. Here is exactly the arithmetic the calculator runs — the Boer men's formula on kilogram and centimetre inputs.
Step 1 — Scale the weight term
The men's formula multiplies weight by 0.407: 0.407 × 80 = 32.56. This is the body-mass contribution to lean mass.
Step 2 — Scale the height term
Next, height is multiplied by 0.267: 0.267 × 180 = 48.06. Taller frames add lean mass through this term.
Step 3 — Add the terms and subtract the constant
Now put it to use. Because his lean mass sits near 77% of weight, David is comfortably inside the healthy 70–90% band. The fat-mass figure (18.6 kg) is what a calorie deficit targets, while the 61.4 kg of lean mass is what resistance training and adequate protein are meant to protect. The next sections show how to read that percentage and how the Boer estimate compares to the James and Hume formulas.
Boer vs James vs Hume formulas
Boer is not the only predictive equation for lean body mass. The two other classics, James (1976) and Hume (1966), use the same inputs — weight, height, and sex — but different coefficients, and they were derived from different populations. They usually land within a kilogram or two of each other for an average adult, then diverge at the extremes of body size.
| Formula | Male equation | Female equation |
|---|---|---|
| Boer (1984) | 0.407 × kg + 0.267 × cm − 19.2 | 0.252 × kg + 0.473 × cm − 48.3 |
| James (1976) | 1.1 × kg − 128 × (kg ÷ cm)² | 1.07 × kg − 148 × (kg ÷ cm)² |
| Hume (1966) | 0.32810 × kg + 0.33929 × cm − 29.5336 | 0.29569 × kg + 0.41813 × cm − 43.2933 |
Weight in kilograms, height in centimetres, result in kilograms. This calculator uses the Boer formula.
- Boer — derived from healthy adults; linear in weight and height. The most widely used clinically and the equation this calculator runs.
- James — uses a squared weight-to-height ratio, so it behaves differently for very heavy or very light people and can under-read lean mass at high body weights.
- Hume — the oldest of the three, derived partly from patients rather than athletes; another linear equation that tends to track close to Boer for mid-range adults.
Lean body mass vs fat-free mass
Lean body mass and fat-free mass are used interchangeably almost everywhere, including on most calculators — but in strict physiology they are slightly different. The distinction is essential fat: the small amount of fat embedded in your bone marrow, organs, and nervous system that your body cannot live without.
| Term | What it includes | Counts essential fat? |
|---|---|---|
| Lean body mass (LBM) | Muscle, bone, organs, water — plus essential fat | Yes (a few percent) |
| Fat-free mass (FFM) | Muscle, bone, organs, water only | No |
Because essential fat is only about 2–5% of body weight in men and 10–13% in women, the gap between the two figures is small — fat-free mass is a touch lower than true lean body mass. In everyday use, fitness apps, and predictive formulas like Boer, the two terms are treated as the same number, and this calculator's output is fine to read as either. If you want the muscularity index built on fat-free mass, the FFMI calculator covers it.
Why lean body mass matters
Lean body mass is not just a vanity metric. It is the figure that several real decisions hinge on, in the clinic and in the gym alike.
Drug dosing
Many medications — including anaesthetic agents and some chemotherapy drugs — are dosed on lean body mass rather than total weight, because fat tissue absorbs these drugs very differently from lean tissue. Dosing a heavier person on total weight alone risks overdosing the active compartment, so clinical pharmacology often uses LBM (frequently via the Boer or James equation) to size the dose safely.
Body composition and metabolism
Lean mass is the metabolically active part of you. The more lean mass you carry, the higher your resting metabolic rate, because muscle and organs burn far more energy at rest than fat does. Tracking LBM rather than weight alone tells you whether a diet is costing you muscle or fat — the whole point of body recomposition.
Protein needs
Protein targets are best set against lean mass, not total weight, since fat tissue has little protein requirement. Basing your daily protein on lean body mass gives a more honest target — particularly for people carrying higher body fat, where total-weight figures would overstate the requirement.
What is a good lean body mass percentage?
LBM is most useful read as a share of your total weight — your lean mass percentage. For most adults a healthy figure sits somewhere between about 70% and 90% of body weight, with men generally higher than women because women carry more essential fat. The flip side is body fat: a higher lean percentage means a lower fat percentage, and vice versa.
| Lean mass as % of body weight | What it suggests |
|---|---|
| below 70% | Higher body fat — more than 30% of weight is fat |
| 70 – 80% | Typical healthy range for many adults |
| 80 – 90% | Lean and athletic — low body fat |
| above 90% | Very lean — competitive-athlete territory (rare) |
General guidance; healthy ranges vary by sex and age, with women healthy at lower lean percentages than men. Not a diagnostic scale.
Because lean percentage and body fat are two sides of the same coin, the most direct way to check this is to measure body fat and subtract from 100%. For that, the body fat calculator estimates your fat percentage from a few tape measurements; whatever is left over is your lean mass percentage.
Lean body mass and FFMI
Lean body mass is an absolute number in kilograms — how much lean tissue you carry, full stop. That makes it hard to compare fairly between a tall person and a short one, because taller people carry more lean mass simply by being bigger. FFMI (Fat-Free Mass Index) solves that by dividing lean mass by height squared, exactly the way BMI works, so it indexes muscularity rather than total quantity.
| Measure | What it reports | Units | Comparable across heights? |
|---|---|---|---|
| Lean body mass | Total lean tissue you carry | Kilograms | No — bigger people score higher |
| FFMI | Lean mass relative to height | Index (kg/m²) | Yes — that is the point |
They are complementary: LBM tells you the absolute quantity (useful for dosing and protein targets), while FFMI tells you how muscular that quantity is for your height (useful for judging a physique against the ~25 natural ceiling). If you have a body-fat figure, feed it into the FFMI calculator to turn your lean mass into a height-adjusted muscularity score.
How to increase lean body mass
Lean mass rises when you give your muscles a reason to grow and the raw material to do it. Unlike fat loss, building lean mass is slow and has a ceiling that tightens each year of training, so consistency matters more than intensity.
- Lift weights with progressive overload. Resistance training is the single strongest driver of lean mass. Aim for 2–4 sessions a week and gradually add weight, reps, or volume over time.
- Eat enough protein. Around 1.6–2.2 g per kg of body weight per day supplies the amino acids muscle is built from and is one of the most reliable ways to add lean mass.
- Eat enough total energy. Building meaningful muscle is easier in a slight calorie surplus; a deep deficit caps how much lean mass you can add at once.
- Be patient with the numbers. Beginners can gain roughly 0.5–1 kg of lean mass a month in the first year; that rate falls to a fraction of that in later years as you approach your potential.
Pair this calculator with the tools that handle the energy side: a maintenance calorie calculator to set your intake, the body fat calculator to watch fat fall as lean mass holds, and the BMI calculator for the weight-for-height baseline.
Lean body mass definitions
Sources and methodology
This calculator estimates lean body mass with the Boer formula (1984): for men, LBM = 0.407 × kg + 0.267 × cm − 19.2; for women, LBM = 0.252 × kg + 0.473 × cm − 48.3, with weight in kilograms and height in centimetres. The James (1976) and Hume (1966) equations are shown for comparison only. Healthy lean-percentage ranges and the essential-fat figures follow standard body-composition references.
Boer P. "Estimated lean body mass as an index for normalization of body fluid volumes in humans." American Journal of Physiology, 1984;247(4):F632–F636.James W. "Research on Obesity: A Report of the DHSS/MRC Group." London, 1976 (James lean body mass equation).Frequently asked questions about the free lean body mass calculator
About this lean body mass calculator
This lean body mass calculator runs entirely in your browser. Your weight, height, and sex never leave your device — nothing is sent to a server, logged, or shared. It applies the Boer formula (1984) to your weight and height in kilograms and centimetres, returning your lean body mass and fat mass, updating instantly on every change.
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