Lean Body Mass Calculator
Calculate lean body mass and fat mass from weight and body fat percentage.
Please note: For general information only. This is not medical advice — speak to a doctor or registered dietitian before changing your diet, training or medication.
What the Lean Body Mass Calculator does
Lean body mass is everything that is not fat: muscle, bone, organs and water. It is the more useful number for setting protein targets and calculating metabolic rate, because fat tissue burns very little energy.
Formula
Fat mass = Weight × Body fat % ÷ 100Lean body mass = Weight − Fat mass
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Units | one of 2 options | Yes | — |
| Weight | kg or lb | Yes | Accepts 1 or more. |
| Body fat percentage | % | Yes | Accepts 1 or more, up to 70. |
How to use it
- Choose Units.
- Enter Weight and Body fat percentage.
- Select Calculate.
Worked example
An 85 kg person at 20% body fat.
- Weight
- 85 kg
- Body fat
- 20
Fat mass 17 kg, lean body mass 68 kg.
Frequently asked questions
Why does lean mass matter for dieting?
It determines your metabolic rate and how strong you stay. The aim of a good diet is to lose fat mass while holding lean mass steady — which needs adequate protein and resistance training.
Method and sources
Method. Fat mass is body weight times body-fat percentage; lean body mass is what remains.
Assumptions
- The body-fat percentage entered is accurate. Everything here inherits its error directly, one for one.
- Lean mass is treated as a single compartment covering muscle, bone, organs and water.
Limitations
- This is arithmetic on a figure you supply, not a measurement. A body-fat percentage from bioimpedance scales can be several points out, and the lean mass moves with it.
- Lean mass includes water, which swings by a kilogram or more day to day with hydration, carbohydrate intake and salt — enough to swamp real change over short periods.
Sources
- None — subtraction from a body-fat percentage the reader supplies — Not applicable. Stated so the missing citation is legible as intentional. The source that matters is whatever produced the body-fat percentage, and that is outside this calculator.