Lean Body Mass and Fat Mass Guide
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Fat mass is body weight multiplied by the estimated body-fat percentage. Lean body mass is the remainder. It includes muscle, bone, organs, connective tissue, and water—not muscle alone.
Obtain the percentage with the Body Fat Calculator, then keep weight and percentage from the same date.
Formulas
body-fat fraction = body-fat percentage ÷ 100
fat mass = body weight × body-fat fraction
lean body mass = body weight − fat mass
The mass units stay the same as the input weight.
Examples
At 180 lb and 21.25% body fat, fat mass is 180 × 0.2125 = 38.25 lb; lean mass is 141.75 lb.
At 80 kg and 25%, fat mass is 80 × 0.25 = 20 kg; lean mass is 60 kg.
Lean mass is not muscle mass
Lean mass changes with hydration, glycogen, gut contents, and measurement error. A short-term increase is not proof of new skeletal muscle.
Rounding rule
Calculate with the unrounded percentage when available and round displayed masses at the end. Displayed components may differ from total weight by a small rounding amount; underlying values should reconcile.
Next steps
- Use How to Track Body Fat Progress before comparing dates.
- Use the Protein Calculator for an adult planning range.
- Use the Calorie Deficit Calculator as a starting estimate with safety guidance.
Sources
Evidence and editorial status
Evidence for this package was checked on September 10, 2026. CalculatorGeek should display a named reviewer or professional credential only after that person has completed and documented the review. This material is educational and is not medical diagnosis, treatment, or individualized nutrition advice.
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