Lean Body Mass Calculator
An 80 kg man at 180 cm has about 61.4 kg of lean body mass with the Boer formula used here. This calculator estimates lean body mass and derived fat mass from weight and height using the sex-specific Boer equations. Enter the formula sex, weight in kilograms, and height in centimeters to see lean mass, fat mass, and fat mass as a percentage of total weight. Lean body mass includes muscle, bone, organs, body water, and every other non-fat component. It is not the same as skeletal muscle mass. The Boer formula is a population equation, not a scan, and it does not use an entered body fat measurement. Its result can differ from the James or Hume formulas and from DEXA, bioelectrical impedance, calipers, or circumference methods.
Quick answer
Lean body mass is your estimated weight minus estimated fat mass.
What this tells you
- •Lean body mass is your estimated weight minus estimated fat mass.
- •The Boer formula uses different constants for men and women.
- •It only needs weight and height, so it skips body fat percentage as an input.
- •The result is a population-based estimate, not a body scan or a diagnosis.
- •Fat mass and fat percentage are derived by subtracting the Boer estimate from total weight.
- •The selected male or female option changes the equation constants and does not diagnose sex or gender.
- •Results display to 1 decimal place, which is calculator rounding rather than measurement precision.
How to Use
- 1Select male or female so the calculator applies the matching Boer formula.
- 2Enter your current body weight in kilograms.
- 3Enter your current height in centimeters.
- 4Calculate to see estimated lean body mass, fat mass, and fat mass percentage.
- 5Track the trend over weeks alongside consistent weigh-ins rather than judging a single result.
- 6Keep the same formula when comparing results over time, since other methods can give different values.
- 7Discuss unexpected body-composition or weight changes with a qualified healthcare professional.
How It Works
Formula
Male: LBM = 0.407 x weight(kg) + 0.267 x height(cm) - 19.2
Female: LBM = 0.252 x weight(kg) + 0.473 x height(cm) - 48.3
Fat mass = weight - LBMThe Boer formula estimates lean body mass directly from weight and height using sex-specific constants, without needing a body fat percentage as an input. Fat mass is simply total weight minus the estimated lean body mass, and fat mass percentage divides that fat mass by total weight. This formula was built from population data and gives a reasonable estimate for most adults, but it is still an approximation rather than a direct measurement.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
Male at 80 kg and 180 cm
The male Boer formula gives 0.407 x 80 + 0.267 x 180 - 19.2, which rounds to 61.4 kg of lean body mass. Fat mass is 80 minus 61.4, or 18.6 kg, which is about 23.2% of total weight.
Female at 65 kg and 165 cm
The female Boer formula gives 0.252 x 65 + 0.473 x 165 - 48.3, which rounds to 46.1 kg of lean body mass. Fat mass is 65 minus 46.1, or 18.9 kg, which is about 29.0% of total weight.
Male at 90 kg and 190 cm
The male equation gives 0.407 x 90 + 0.267 x 190 - 19.2 = 68.16 kg. The unrounded difference is 21.84 kg of estimated fat mass, or about 24.27% of total weight.
Female at 55 kg and 160 cm
The female equation gives 0.252 x 55 + 0.473 x 160 - 48.3 = 41.24 kg. Subtracting from 55 kg gives 13.76 kg of estimated fat mass, or about 25.02%.
Same measurements with different formula sex
With the female Boer constants, 0.252 x 80 + 0.473 x 180 - 48.3 gives 57.0 kg. This differs from the 61.4 kg male result for the same height and weight because the equations were fitted separately.
What the Result Represents
The output separates estimated total weight into two broad components.
| Output | Calculation | Included |
|---|---|---|
| Lean body mass | Boer sex-specific equation | Muscle, bone, organs, water, and other non-fat tissue |
| Fat mass | Total weight minus estimated lean mass | Estimated fat component |
| Fat mass percent | Fat mass divided by weight x 100 | Derived percentage of total weight |
| Method | Male or female Boer equation | Formula label, not a clinical classification |
None of these outputs directly measures a tissue compartment. They all depend on the Boer estimate.
Why lean mass estimates vary by method
The Boer formula predicts lean body mass from only weight, height, and one of two sex-specific coefficient sets. The male equation gives more weight to total body weight, while the female equation gives more weight to height. These coefficients came from fitted population data rather than a physical law.
Other equations use different coefficients and source populations. The James and Hume methods can therefore produce different lean mass estimates for the same adult. A body fat based method first estimates fat percentage and subtracts fat mass, introducing the assumptions and measurement error of that body fat method instead.
Lean mass is sensitive to fluid balance. Hydration, glycogen, edema, recent food intake, and illness can change measured weight without representing a comparable change in muscle or fat. Since the Boer equation uses scale weight directly, those changes can move the output.
For tracking, use similar weighing conditions and keep the method fixed. A gradual pattern may be useful as context, but the calculator cannot tell whether a change came from muscle, water, organs, bone, or another lean component.
Common mistakes
- Entering pounds as kilograms or inches as centimeters
- Selecting the wrong sex option, which changes the formula constants and the result
- Treating one lean body mass estimate as a precise or medical measurement
- Comparing results with tools that use a different formula, such as the Boer, James, or Hume methods
- Reading lean body mass as muscle mass even though it includes water, bone, and organs
- Assuming the derived fat percentage is a direct body fat measurement
- Comparing weigh-ins taken under very different hydration or meal conditions
- Using adult equation results to assess a child, teenager, or pregnancy
Limitations
This calculator uses only the Boer formula and does not average multiple methods. It estimates lean mass from weight, height, and binary sex-specific coefficients. It does not account for age, body frame, muscle distribution, hydration, edema, ethnicity, disability, medication effects, recent weight change, athletic training, or pregnancy. Results may be less reliable for very muscular or very lean adults, older adults, people outside the formula's source population, and people with fluid-balance changes. It is not intended for children or teenagers. Derived fat mass and fat percentage inherit every limitation of the lean mass estimate.
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