How to Calculate Lean Body Mass: Formulas and Why It Matters
Lean body mass is everything in your body that is not fat. Here is how to estimate it with the Boer formula and why the number is worth tracking.
Lean body mass is everything in your body that is not fat: muscle, bone, organs, and the water they hold. It matters because most of what makes you strong, resilient, and metabolically active lives in that lean tissue. When people lose weight, the goal is usually to lose fat while keeping lean mass, and you cannot manage what you do not measure. Estimating lean body mass gives you a number to protect.
The simple way if you know body fat
If you already have a body fat percentage, lean body mass is just subtraction. Multiply your weight by your body fat percentage to get fat mass, then subtract that from your weight. For example, at 80 kilograms with 20 percent body fat, fat mass is 16 kilograms, so lean body mass is 64 kilograms. This is the most direct method, but it is only as accurate as your body fat estimate.
- Fat mass equals weight multiplied by body fat percentage.
- Lean body mass equals weight minus fat mass.
- Accuracy depends entirely on the body fat reading you start with.
- Use the same body fat method each time so the trend is meaningful.
The Boer formula
If you do not have a body fat measurement, the Boer formula estimates lean body mass from height, weight, and sex. For men it is 0.407 times weight in kilograms plus 0.267 times height in centimeters minus 19.2. For women it is 0.252 times weight plus 0.473 times height minus 48.3. It is only an estimate built from population averages, so a very muscular or very lean person will not fit it perfectly, but it is a reasonable starting point when no scan is available.
- Boer, men: 0.407 x weight kg + 0.267 x height cm - 19.2.
- Boer, women: 0.252 x weight kg + 0.473 x height cm - 48.3.
- Needs only height, weight, and sex.
- Based on population averages, so extremes fit less well.
Why the number is useful
During a fat loss phase, watching lean body mass tells you whether you are losing the right kind of weight. If lean mass holds steady while total weight drops, you are losing mostly fat, which is the goal. Lean mass also helps set protein and calorie targets, since needs scale with lean tissue rather than total weight. Pair it with resistance training, which is the main way to protect and build lean mass as you age.