Lean Body Mass Calculator
Separate estimated fat and fat-free mass using body weight and an existing body-fat percentage.
Your estimate
Enter your values and calculate. Results will appear here.
This is the common calculator interpretation of lean body mass as fat-free mass. It includes more than muscle: water, organs, bone and other non-fat components are included. Strict body-composition terminology can distinguish lean body mass from fat-free mass.
How to use this calculator
Use a body-fat percentage you already have and enter its numeric percent, such as 20 rather than 0.20. Select weight units and calculate. Record the method behind that percentage if you compare results over time.
Formula and methodology
Estimated fat mass = body weight × body fat % / 100. Fat-free mass = body weight − estimated fat mass.
Worked example
At 70 kg and 20% body fat, estimated fat mass is 14 kg and fat-free mass is 56 kg. If the percentage were 25% instead, the same body weight would give 17.5 kg fat and 52.5 kg fat-free mass.
What the result means
This is the common calculator interpretation of lean body mass as fat-free mass. It includes more than muscle: water, organs, bone and other non-fat components are included. Strict body-composition terminology can distinguish lean body mass from fat-free mass.
Limitations
The arithmetic is exact for the inputs, but its real-world accuracy is limited by the body-fat estimate. Hydration and measurement conditions can affect body-composition readings, and this tool cannot isolate muscle tissue.
Common mistakes
Do not label the fat-free result as muscle mass. Also avoid comparing two results based on different body-fat measurement methods as if their difference were necessarily a real tissue change.
Frequently asked questions
Is fat-free mass the same as muscle mass?
No. It includes water, organs, bone and other non-fat tissue as well as muscle.
Can I use the body-fat calculator result?
Yes, as an illustration. Its prediction error carries through to both mass estimates.