Non-fat mass and implied fat mass
Lean mass here means the non-fat portion of body weight, not muscle alone. The Boer mode estimates it from height and weight; known-percentage mode calculates it directly from your supplied percentage. Results use the selected weight unit.
For a separate adult BMI-based fat-percentage estimate, use the Body Fat Calculator .
The formula
Boer mode uses sex-specific linear equations. The remaining weight is reported as implied fat mass, and fat / total × 100 supplies the implied percentage. Known-percentage mode uses your entry directly. Conversion uses 0.45359237 kg/lb and 2.54 cm/in; outputs are rounded only for display.
Two methods for a 70 kg adult
For 70 kg, 170 cm, and male coefficients, Boer gives 0.407 × 70 + 0.267 × 170 − 19.2 = 54.68 kg. Implied fat mass is 15.32 kg, or 21.89%. With a supplied body-fat value of 20%, the other method instead gives 56 kg non-fat mass and 14 kg fat mass.
How to use this calculator
- Choose the Boer estimate or known-body-fat method.
- Enter body weight in the selected unit, then fill the fields shown for your method.
- Calculate and compare the method label, mass breakdown, and implied fat percentage.
Lean mass includes more than muscle
The two-part calculation splits total body weight into fat and the remaining mass. That remainder includes more than skeletal muscle, so it cannot be read as a measure of muscle gain. The calculator also does not separately estimate water, bone, or organ mass. Keep that distinction when comparing a result with a report that defines its body-composition compartments differently.
Choose the method to match your evidence
The Boer equation needs height, weight, and the published sex coefficients. It supplies an estimate when no measured body-fat input is available. The known-percentage method answers an arithmetic question: how much weight remains after the stated fat fraction is removed? It does not validate the percentage or improve the method that produced it. Switching methods can change the result because the assumptions change, even when scale weight stays the same.
Check units and implausible estimates
Changing a unit changes how the entered number is interpreted, so replace the value with the equivalent measurement. Calculations are done in kilograms and centimeters before converting back for display. Boer outputs at or below zero, or above total body weight, are rejected as incompatible with this two-part model. The calculator does not silently clamp these results. It is an adult educational estimate, with no drug dosing, clinical assessment, or individual nutrition prescription.
Assumptions & limitations
What this calculation assumes
- Boer mode follows the published male/female coefficient equations.
- Known-percentage mode assumes that the entered percentage and body weight refer to the same measurement context.
What to keep in mind
- Boer model results must be greater than zero and no greater than total body weight.
- The supplied-fat mode accepts 0–99%; uncertainty in that percentage carries into the result. Neither method measures skeletal muscle.
- Not intended for pediatric, pregnancy, or medication-dosing use.
Common questions
Does lean body mass mean muscle mass?
No. This page reports the non-fat portion of total weight. It does not isolate skeletal muscle.
Why do Boer and a measured fat percentage give different values?
Boer is a height-and-weight model; the other method uses your supplied percentage. They are different sources of information and need not agree.
Does the calculator keep my entered percentage when I change modes?
Yes within the current page session. Returning to known-percentage mode restores your entry; Reset returns to the example values.