What this calculator does
Get an estimate of how much of your weight is lean mass, meaning everything that isn't fat. Lean body mass (LBM) is your total weight minus your body fat. It's your muscle, bone, organs, skin and water. It matters because lean tissue burns most of the energy your body uses at rest. Some medical calculations also use it instead of total weight.
Measuring LBM directly takes special equipment, such as a DEXA scan (a low-dose X-ray body scan). This calculator estimates it instead, from your sex, height and weight, using three published formulas. It also shows the body fat percentage the main estimate suggests. These figures describe an average adult with your measurements. They're not a measurement or a diagnosis. For a look at your own body makeup, talk with a healthcare professional.
Where it's used
Why this math matters outside the classroom:
- Boer formula. For a man of 80 kg and 180 cm, Boer gives 0.407 × 80 + 0.267 × 180 − 19.2, which is about 61.4 kg.
- Unit conversion. Converting 176 lb to about 79.8 kg and 70 in to 177.8 cm comes before the formulas, which use metric units.
- Comparing formulas. Students run Boer, James and Hume on the same inputs to see how three published equations differ by a few kilograms.
- Strength tracking. People who lift weights note lean mass estimates next to their weight to see more of the picture over time.
- Nutrition planning. Some nutrition plans express daily protein per kilogram of lean mass rather than per kilogram of total weight.
Grade level and standards: Grades 6–7; Common Core State Standards for Mathematics 6.EE.A.2.c. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.
How to use it
- Choose US units (feet, inches, pounds) or metric units (centimeters, kilograms).
- Select your sex and enter your height and weight.
- Read the Boer estimate, shown first, and compare it with the James and Hume results.
- If you have your own body fat estimate, the body fat calculator gives a lean mass figure from tape measurements instead.
What the inputs mean
- Sex
- Each formula has a separate version for men and for women, built from data on each group.
- Weight
- Your current body weight. The formulas use only height and weight, so they can't tell muscle from fat.
Formula and method
Boer (male) = 0.407 × W + 0.267 × H − 19.2 Boer (female) = 0.252 × W + 0.473 × H − 48.3 James (male) = 1.1 × W − 128 × (W ÷ H)² James (female) = 1.07 × W − 148 × (W ÷ H)² Hume (male) = 0.32810 × W + 0.33929 × H − 29.5336 Hume (female) = 0.29569 × W + 0.41813 × H − 43.2933 Implied body fat % = (1 − LBM ÷ W) × 100
In the formulas, W is your weight in kilograms, H is your height in centimeters, and LBM is the lean body mass result. Results come out in kilograms. For US units they're changed to pounds (1 lb = 0.45359237 kg exactly). US heights are changed with 1 in = 2.54 cm.
Hume (1966) and James (1976) are older formulas. Boer (1984) came later and is often preferred, so it's shown as the main result. The James formula has a known flaw. It subtracts an amount that grows with the square of your weight. So once weight is high for your height, its estimate starts to drop. If any formula gives zero, a negative number, or more than your weight, it's marked as not meaningful.
Worked example
A man is 5 ft 10 in tall and weighs 180 lb.
- Change to metric: 177.8 cm and 81.647 kg.
- Boer: 0.407 × 81.647 + 0.267 × 177.8 − 19.2 = 61.50 kg = 135.6 lb.
- James: 1.1 × 81.647 − 128 × (81.647 ÷ 177.8)² = 62.82 kg = 138.5 lb.
- Hume: 0.32810 × 81.647 + 0.33929 × 177.8 − 29.5336 = 57.58 kg = 126.9 lb.
- Body fat this suggests (Boer): (1 − 61.50 ÷ 81.647) × 100 = 24.7%.
Result: His estimated lean body mass is about 135.6 lb (Boer). The three formulas range from 126.9 to 138.5 lb.
Understanding the result
For people of average build, the three formulas usually land within a few kilograms of each other. A wider gap suggests your measurements are near the edge of what the formulas handle well.
The only inputs are height and weight. So two people with the same measurements get the same answer, even if one is far more muscular. The body fat percentage has the same limit, so don't read it as a measurement.
For a better personal estimate, pair your weight with a body fat percentage from a more direct method. If the results matter for a health decision, talk them over with a healthcare professional.
Assumptions and limitations
- You're an adult with a typical body makeup for your height and weight.
- Your measurements are accurate. Results on screen are rounded to one decimal place, but the math behind them isn't.
- The formulas came from fairly small groups of adults. They may be less reliable for groups not well represented in that data and for older adults. The same goes for people with very high or very low body fat or muscle.
- They can't tell muscle from fat. So they tend to underestimate very muscular people and overestimate people with obesity.
- The James formula becomes unreliable when weight is high for height.
- Not suitable for children, teens or during pregnancy.
Frequently asked questions
Which lean body mass formula should I use?
Boer is shown first. Comparing all three is still useful. Where they agree, the estimate is more trustworthy for someone of typical build.
Is lean body mass the same as muscle mass?
No. Lean body mass includes everything that isn't fat: muscle, but also bone, organs, skin, blood and water. Muscle is only one part of it.
Why does the James result say it's not meaningful?
The James formula subtracts an amount based on the square of weight divided by height. When weight is high for a given height, that amount grows faster than the rest. So the result can drop below zero or stop making sense. The calculator hides it rather than show a misleading number.
How is this different from the body fat calculator?
The body fat calculator uses tape measurements of your neck, waist and hips, which reflect your actual shape. This calculator uses only height and weight. So it describes an average person with your measurements, not you in particular.
How this calculator is checked
Method: Boer P (1984), James WPT (1976) and Hume R (1966) lean body mass equations (weight in kg, height in cm); unit conversions use the international inch and pound. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.
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