A familiar situation
Separate estimated fat mass and lean mass when a body-fat estimate is already available.
How to use this calculator
- Enter weight, estimated body fat.
- Select Calculate to see the answer and its breakdown.
- Keep a scenario, change an input and compare the results.
Use consistent units and check the assumptions below before applying the result.
How lean body mass is calculated
Fat mass = body weight × body-fat fraction; lean body mass = body weight − fat mass. Because the input percentage is itself an estimate, any error in that percentage carries directly into the lean-mass result. At 80 kg and an estimated 20% body fat, lean mass is 80×0.80 = 64 kg.
Lean mass=weight×(1−body-fat fraction).At 80 kg and an estimated 20% body fat, lean mass is 80×0.80 = 64 kg.
Assumptions & useful context
Lean mass includes water, bone, organs, and muscle; it is not skeletal muscle mass alone.
Health calculations are estimates for education. They cannot diagnose a condition or replace a clinician’s assessment. Individual needs vary, especially during pregnancy, illness and growth.
Frequently asked questions
Is lean mass the same as skeletal muscle?
No. Lean mass includes water, organs, bone, and other non-fat tissue.
How accurate is the output?
It depends directly on the accuracy of the body-fat percentage you enter.