Estimate the weight of everything in your body that isn't fat — muscle, bone, organs, and water — from your height and weight.
Estimate by formula:
| Formula | Lean body mass |
|---|
These are estimates from population formulas using only height and weight, so they're less precise than methods like DEXA. The headline figure uses the Boer formula. This tool is for educational purposes only and is not medical advice.
Lean body mass, sometimes called fat-free mass, is the combined weight of everything in your body that isn't fat: muscle, bone, organs, connective tissue, and water. Your total weight is simply lean body mass plus fat mass, so once you estimate one, the other follows.
This calculator uses established equations that estimate LBM from your height, weight, and sex. It then derives your fat mass and body fat percentage from that figure.
Three well-known equations appear in the results. The Boer formula is widely used and serves as the headline estimate here. The James and Hume formulas are alternatives developed from different population data; comparing them shows how much the estimate can shift depending on the method.
All three rely only on height, weight, and sex, which makes them quick but approximate — they can't see your actual muscle or fat, only predict a typical value for your measurements.
LBM shows up in several places. In medicine, some drug doses are calculated from lean mass rather than total weight, because fat tissue affects how a drug distributes. In fitness, tracking LBM helps confirm whether weight changes are coming from muscle or fat.
It's also the basis for several nutrition targets — protein needs and resting metabolism both track more closely with lean mass than with total body weight.
When you're dieting or training, the goal is usually to lose fat while keeping lean mass, or to add lean mass while limiting fat. Watching LBM alongside total weight tells you which is happening — losing weight while LBM holds steady means you're mostly losing fat, which is what you want.
Because the formula-based estimate moves with your weight, pair it with a body-fat measurement for a clearer read on real composition changes.
Formula estimates are convenient but not exact. They're built from population averages, so an individual with unusually high or low muscle for their size will be off. Methods like DEXA scans, hydrostatic weighing, or air-displacement are far more accurate when precision matters.
For everyday tracking, the formula is fine as long as you use it consistently and watch the trend rather than treating a single number as definitive.
It's the weight of everything in your body that isn't fat — muscle, bone, organs, and water. Total weight equals lean body mass plus fat mass.
This tool uses the Boer, James, and Hume formulas, which estimate LBM from height, weight, and sex. Fat mass and body fat percentage are then derived from that figure.
Muscle is part of lean body mass, but LBM also includes bone, organs, and water. So LBM is always larger than skeletal muscle mass alone.
Each was developed from a different population, so they weight height and weight slightly differently. Seeing all three gives you a realistic range rather than one false-precise value.
Resistance training combined with adequate protein and enough total calories builds muscle, the main component of lean mass you can change. Progress is gradual.