Lean body mass calculator
Calculate estimated lean mass and fat mass from your body weight and body-fat percentage. Simple arithmetic, with useful context.
Your details
Use a recent estimate from a consistent measurement method.
Body weight minus the estimated mass of body fat.
136.0lb
34.0lb
Lean mass includes more than muscle.
Water, bone, organs and other non-fat tissue are included. A change in this estimate does not, by itself, show muscle gain or loss.
The result inherits any uncertainty in your body-fat measurement. It is an estimate, not a clinical body-composition assessment.
The numbers, explained.
Our methodology ↗This tool separates body weight into estimated fat mass and the remaining mass. Enter your weight and a body-fat percentage from your chosen measurement method. The calculation itself is straightforward; the quality of the input is the limiting factor.
For a 170 lb person at 20% body fat, the arithmetic gives 34 lb of fat mass and 136 lb of non-fat mass. Changing the percentage to 22% with the same weight gives 132.6 lb of non-fat mass. That change in the estimate does not establish a change in actual muscle tissue.
We use “lean body mass” as a familiar label for this non-fat remainder, often called fat-free mass in a two-compartment calculation. More specialized body-composition models distinguish these terms more carefully.
If you do not have a body-fat estimate, the historical Navy calculator can provide a tape-based starting point. Keep the same method when comparing results over time and remember that every derived mass inherits its uncertainty.
| At 170 lb body weight | Lean mass estimate |
|---|---|
| 15% body fat | 144.5 lb |
| 20% body fat | 136.0 lb |
| 25% body fat | 127.5 lb |
These examples illustrate arithmetic only. They are not targets or health categories.
A little more clarity.
Can this tell me my muscle mass?
No. The non-fat remainder includes much more than skeletal muscle. It cannot separate muscle from water, bone or organs.
Why is my result different from a smart scale?
A scale may estimate body fat with bioelectrical impedance and its own model. This tool simply uses the percentage you provide. Entering exactly the same percentage and weight should give the same basic two-compartment arithmetic.
How much does an uncertain body-fat percentage matter?
At 170 lb, a one-percentage-point change shifts the lean-mass estimate by 1.7 lb. That is why small changes in the displayed result can reflect input uncertainty rather than tissue change.
Why are there no ideal lean-mass targets?
This arithmetic cannot determine an appropriate body-composition target. A target would require personal context beyond the two inputs used here.
Keep exploring.
All calculators ↗Methods checked against the linked references · September 14, 2026. Educational estimates for adults. No individual medical assessment.