Lean Body Mass Calculator
Estimate your lean body mass and fat mass from height, weight, and sex using three validated formulas — Boer, James, and Hume — with the fat-free percentage.
Lean body mass (LBM) is your total weight minus your body fat — muscle, bone, organs, and water combined. It’s a more useful number than bodyweight alone because it’s what determines how many calories you burn at rest and how much protein your body needs.
The three formulas
Because you can’t weigh your lean mass directly without a scan, this calculator estimates it from your height, weight, and sex using three published equations:
- Boer (1984) — the modern default, validated for normalizing body-fluid volumes; the most reliable for a typical adult.
- James (1976) — a widely used clinical equation.
- Hume (1966) — one of the earliest height-and-weight predictions, still referenced in pharmacology.
We show all three plus the fat mass (your weight minus lean mass) and your lean percentage, so you get a realistic band rather than one false-precision figure.
For children aged 13 and under, the calculator automatically switches to the Peters (2011) equation instead. Boer, James, and Hume were derived in adults and aren’t valid for young children, whereas Peters was developed specifically for the pediatric age range and doesn’t split by sex.
How to read your result
A higher lean percentage generally reflects more muscle and less fat. During a diet, the goal is to lose weight while keeping LBM steady — that’s the sign the loss is coming from fat, not muscle. Pair this with a body fat calculator for the same information from the other direction.
When a formula isn’t enough
Height-and-weight formulas assume an average body composition, so they can be off for very muscular, very lean, or older adults. If you have a measured body-fat percentage (from DEXA, hydrostatic weighing, calipers, or the U.S. Navy tape method), lean mass = weight × (1 − body-fat fraction) will be more accurate than any estimate here.
Frequently asked questions
What is lean body mass?
Lean body mass (LBM) is everything in your body that isn't fat — muscle, bone, organs, connective tissue, and body water. It typically makes up about 60–90% of body weight, and it's the part that burns most of your calories at rest.
How is lean body mass different from muscle mass?
Muscle is only one component of lean mass. LBM also includes your skeleton, organs, and the water in your body, so it is always larger than muscle mass alone. There's no simple formula for skeletal muscle by itself, which is why LBM is the figure most tools estimate.
Which lean body mass formula is most accurate?
The Boer formula is generally the most reliable for the average adult, so we show it as the headline. James and Hume are shown alongside for comparison. For lean or very muscular people, a direct body-fat measurement (DEXA, Navy tape, calipers) is more accurate than any height-and-weight formula.
Why does lean body mass matter?
LBM drives your resting metabolic rate, so it's used to set protein targets, dose certain medications and anaesthetics, and track whether weight loss is coming from fat rather than muscle. The Katch-McArdle BMR formula is built directly on lean mass.