Lean Body Mass Calculator
Calculate lean body mass from your weight and body fat percentage, or compare Boer, James, and Hume estimates from height and weight.
Lean body mass calculator
Advertisement
How to Calculate Lean Body Mass
Lean body mass is body weight minus fat mass. When you already know your body fat percentage, you do not need height, sex, or a regression equation.
Lean body mass = body weight × (1 − body fat percentage / 100)
At 80 kg and 20% body fat, fat mass is 16 kg and lean body mass is 64 kg. The lean share is 80%. The same arithmetic in pounds: 180 lb at 20% body fat is 144 lb lean and 36 lb fat. That result is only as accurate as the percentage you entered.
Lean Body Mass Formulas Explained
Use the body-fat method when you have a percentage. Use Boer, James, or Hume only when you do not. All three regressions need weight in kilograms and height in centimeters. Each has separate male and female coefficients from adult samples. They estimate a typical body at that size. They do not see your muscle or fat, and this page does not average them.
| Method | Inputs | Equation | Limit |
|---|---|---|---|
| Body fat % | Weight and body fat percentage | Weight × (1 − body fat % / 100) | Only as good as the percentage |
| Boer (1984) | Adult sex group, height, weight | Male: 0.407W + 0.267H − 19.2 Female: 0.252W + 0.473H − 48.3 | Linear size estimate, not a scan |
| James (1976) | Adult sex group, height, weight | Male: 1.1W − 128(W/H)² Female: 1.07W − 148(W/H)² | Falls after about BMI 43 (male) or 36 (female) |
| Hume (1966) | Adult sex group, height, weight | Male: 0.32810W + 0.33929H − 29.5336 Female: 0.29569W + 0.41813H − 43.2933 | Older linear estimate; can fall outside 0 to body weight |
W is kilograms and H is centimeters. The Hume male height coefficient is 0.33929.
How to Use This Calculator
- Use body fat percentage when you have one. Choose “I know my body fat %,” enter weight in pounds or kilograms, and enter the percentage. Height and sex are not required.
- Or estimate from height and weight. Choose “Estimate from height and weight,” select the male or female coefficient group, and enter height and weight. US units use feet and inches.
- Read the primary result and the comparison. Body-fat mode shows lean mass, fat mass, and the lean share. Height-and-weight mode shows Boer as the labeled primary estimate, with James and Hume beside it. The formulas are not averaged.
- Treat the number as an estimate. A direct result is only as good as the body fat percentage. Boer, James, and Hume cannot see muscle or fat. Do not use either result for medication dosing or for a child.
US units are the default: pounds, and feet plus inches in the height-and-weight mode. Metric uses kilograms and centimeters. Switching units converts the current measurements. It does not reuse the same digits as a different unit. Results are shown in both pounds and kilograms.
Lean Body Mass Calculation Examples
Example 1 — 80 kg and 20% body fat
- Lean body mass: 64 kg
- Fat mass: 16 kg
- Lean share: 80%
Example 2 — 180 lb and 20% body fat
- Lean body mass: 144 lb (65.32 kg)
- Fat mass: 36 lb (16.33 kg)
Example 3 — male coefficients, 80 kg and 180 cm
The same inputs produce three estimates. Rounded to the hundredth of a kilogram: Boer 61.42 kg, James 62.72 kg, and Hume 57.79 kg. Before that rounding, James is about 62.716 kg and Hume is about 57.787 kg. The equations were fitted on different samples, so identical inputs still produce different results.
The female coefficients at the same 80 kg and 180 cm are Boer 57 kg, James 56.37 kg, and Hume 55.63 kg. Sex in these equations is the research group used to fit the coefficients.
Lean Body Mass vs Muscle Mass
Lean body mass contains more than skeletal muscle. Bone, organs, skin, connective tissue, and water are still in the number after fat mass is removed. A search for a lean muscle mass calculator usually wants skeletal muscle. This page does not estimate that. Calling total lean weight “muscle” overstates how much of it can change with training.
Lean Body Mass vs Fat-Free Mass
In gym and calculator use, lean body mass and fat-free mass both mean weight minus fat. Technical definitions can split them: fat-free mass excludes all lipid, while some lean-body-mass definitions keep a small amount of essential fat in cell membranes and organs. Published Boer, James, and Hume equations do not make that split. If you need a lab compartment, use the method named on the report, such as a DEXA lean-tissue result, instead of treating these equations as the same test.
Which Lean Body Mass Formula Should You Use?
Start with a measured body fat percentage when you have one you trust. The subtraction does not assume an average body. If the percentage came from a consumer scale or a quick tape estimate, say so when you read the lean mass: the arithmetic will not correct a weak input.
If you have no percentage, compare Boer, James, and Hume. Boer is the labeled primary estimate because it is a commonly cited linear adult equation. A laboratory scan can still disagree with it. James can rise and then fall as weight increases, around BMI 43 with the male coefficients and around BMI 36 with the female coefficients, so a heavier input can produce a smaller James result. Hume is the oldest of the three and can return a value below zero or above body weight at extreme sizes. Those outputs are shown and marked unreliable. The calculator leaves the raw equation result visible.
All three were developed for adults. Do not apply them to children. They are also the wrong tool for medication dosing. A clinician who needs lean body weight for a drug protocol should use the equation and measurement that protocol specifies.
Common Lean Body Mass Calculation Mistakes
- Reading lean body mass as if it were skeletal muscle.
- Leaving the unit on kilograms after typing a pound weight, or the reverse.
- Entering height in meters in Boer, James, or Hume. Those equations expect centimeters.
- Treating an estimated body fat percentage as if it were a DEXA result.
- Assuming a regression formula is exact because it shows a decimal.
- Using adult equations for a child.
- Using any of these results to choose a medication dose.
Frequently Asked Questions
How do I calculate my lean body mass?
If you know your body fat percentage, multiply body weight by 1 minus that percentage divided by 100. At 180 lb and 20% body fat, lean body mass is 144 lb and fat mass is 36 lb. At 80 kg and 20% body fat, lean body mass is 64 kg and fat mass is 16 kg.
Can I calculate lean body mass without knowing my body fat percentage?
You can estimate it. Boer, James, and Hume use adult height, weight, and sex-specific coefficients. They describe a typical body at that size. They do not know how much of your weight is muscle or fat, so the three results differ.
Is lean body mass the same as muscle mass?
No. Lean body mass includes skeletal muscle, bone, organs, connective tissue, and water. A lean-mass figure is larger than skeletal muscle alone. This calculator does not measure muscle.
What is the difference between lean mass and fat-free mass?
People often use the terms for the same idea: body weight minus fat. In research, fat-free mass usually excludes all lipid, while some definitions of lean body mass still include a small amount of essential fat in organs and the nervous system. The gap is smaller than the spread between Boer, James, and Hume. This page labels each result by the method that produced it.
Can I calculate lean body mass in pounds?
Yes. Enter weight in pounds, or switch the calculator to US units for feet and inches. The body-fat method multiplies the pound weight directly. Boer, James, and Hume convert pounds to kilograms and inches to centimeters, then convert the result back to pounds.
Which is better: Boer, James, or Hume?
None of them is the best measurement of you. They were built from different adult samples, for different purposes, decades apart. Boer is shown first because it is a widely cited linear equation, not because it is always closest to a scan. If you have a reliable body fat percentage, use that instead of picking the equation you prefer.
How accurate is an online lean body mass calculator?
A body-fat calculation is exact arithmetic and only as accurate as the percentage you type in. A home scale, a tape estimate, and a DEXA report can disagree by several percentage points, and each point moves lean mass. Height-and-weight formulas add a second limit: they cannot tell two people of the same size apart. Do not treat the output as a clinical measurement.
Does lean body mass include water and bone?
Yes. Lean body mass is body weight minus fat mass, so bone, organs, and body water stay in the lean figure. That is why it should not be read as muscle weight.
Why do different lean mass formulas give different results?
Boer, James, and Hume were fitted on different people with different measurement methods. For a male at 80 kg and 180 cm, Boer is 61.42 kg, James is about 62.72 kg, and Hume is about 57.79 kg. The spread is the uncertainty of estimating body composition from size alone.
Does losing body weight mean losing lean body mass?
Not necessarily. Weight loss can be fat, lean tissue, water, or a mix. A lower scale weight does not tell you which. Repeating the same method over time is more informative than switching formulas between check-ins.
Can BMI tell me my lean body mass?
No. BMI is weight divided by height squared. It does not separate fat from lean tissue. Two adults with the same BMI can have different lean mass. Use a body fat percentage for the direct calculation, or read Boer, James, and Hume as size-based estimates only.
Does a body fat percentage measurement improve the calculation?
It changes the calculation from a size-based estimate to direct subtraction. The lean mass is then only as good as that percentage. A careful measurement is more specific to you than Boer, James, or Hume. A rough percentage still produces a rough lean mass.
Related Calculators
- Body fat calculatorEstimate a body fat percentage from tape measurements, then use that percentage here.
- BMI calculatorAdult height-and-weight screening. BMI does not measure lean mass or fat mass.
- Calorie calculatorEstimate daily calories from age, sex, height, weight, and activity.
- Ideal weight calculatorCompare height-based ideal weight formulas and a healthy BMI weight range.
- Macro calculatorSplit a calorie target into protein, carbohydrate, and fat.
- Water intake calculatorEstimate daily ounces and cups from the same body-weight context.
Sources and limitations
This calculator is for education and fitness tracking. It is not a medical device, not a body-composition test, and not for medication dosing or clinical decisions. A DEXA scan or another laboratory method measures tissue compartments directly. These equations do not.
- Hume R. Prediction of lean body mass from height and weight. J Clin Pathol. 1966;19(4):389-391. PubMed 5929341. Male equation used here: 0.32810W + 0.33929H − 29.5336.
- Boer P. Estimated lean body mass as an index for normalization of body fluid volumes in humans. Am J Physiol. 1984;247(4 Pt 2):F632-F636. PubMed 6496691.
- James WPT. Research on Obesity: a report of the DHSS/MRC group. London: Her Majesty’s Stationery Office; 1976.
- The optional Katch–McArdle illustration is 370 + 21.6 × lean mass in kilograms. It is not a measured metabolic rate and not a calorie prescription.
Inputs are calculated in your browser and are not uploaded. Last reviewed 2026-10-09.