Lean body mass (LBM) is everything in your body that isn't fat: muscle, bone, organs, connective tissue, and water. It's a useful number because most of your metabolism, strength, and physical resilience track with lean mass rather than total body weight. Two people at the same weight can have very different builds.
This lean body mass calculator estimates your fat-free mass from just your weight, height, and sex, and also implies your approximate body fat percentage. Knowing your LBM helps you set realistic muscle-gain or fat-loss goals and better understand your daily calorie and protein needs.
How the lean body mass calculator works
This calculator uses the Boer formula, a widely cited equation that estimates lean body mass from weight and height. There are two versions, one for each sex, because men and women carry fat and lean tissue in different proportions:
- Men: LBM = 0.407 x weight(kg) + 0.267 x height(cm) - 19.2
- Women: LBM = 0.252 x weight(kg) + 0.473 x height(cm) - 48.3
The result is your lean (fat-free) mass in kilograms, which the tool also converts to pounds. From there it implies your fat mass (total weight minus LBM) and your approximate body fat percentage: (weight - LBM) / weight x 100.
You need just three inputs: your current weight, your height, and your sex. The calculator accepts both metric (kg, cm) and US units (lb, ft/in) and converts automatically, updating the moment you change a value. Because the formula relies only on height and weight, it estimates LBM for an average build rather than measuring it directly.
Worked example
Example (man, 80 kg, 180 cm):
LBM = 0.407 x 80 + 0.267 x 180 - 19.2 LBM = 32.56 + 48.06 - 19.2 LBM = 61.42 kg (about 135.4 lb)
Fat mass = 80 - 61.42 = 18.58 kg, so his approximate body fat is 18.58 / 80 x 100 = 23.2%.
For a woman at 65 kg and 165 cm: LBM = 0.252 x 65 + 0.473 x 165 - 48.3 = 16.38 + 78.05 - 48.3 = 46.13 kg, an implied body fat of about 29%.
Things to keep in mind
- The Boer formula is an estimate based only on height, weight, and sex. It does not measure your tissue directly and can be off for very muscular, very lean, or higher-body-fat individuals.
- It assumes an average build. Bodybuilders and elite athletes will have their LBM underestimated, while those with high body fat may have it overestimated.
- For an accurate measurement, methods like DEXA scans, hydrostatic weighing, or BodPod are far more precise than any height-weight equation.
- The implied body fat percentage is a by-product of the LBM estimate, not a direct reading, so treat it as a rough guide rather than a clinical figure.
Frequently asked questions
What is lean body mass?
Lean body mass (LBM) is your total body weight minus all of your fat. It includes muscle, bone, organs, skin, connective tissue, and the water in your body. It's sometimes called fat-free mass and typically makes up the majority of an adult's body weight.
What is a good lean body mass percentage?
For most men, lean mass is around 75-85% of body weight (15-25% body fat); for most women it's roughly 70-80% (20-30% body fat). Athletes carry higher lean percentages. There's no single ideal, since it depends on sex, age, and training goals.
Which formula does this lean body mass calculator use?
It uses the Boer formula: for men, LBM = 0.407 x weight(kg) + 0.267 x height(cm) - 19.2; for women, LBM = 0.252 x weight(kg) + 0.473 x height(cm) - 48.3. The Boer equation is one of the most commonly referenced LBM formulas in clinical and fitness settings.
How is lean body mass different from body fat?
They're two halves of your weight. Lean body mass is everything that isn't fat (muscle, bone, water, organs), while fat mass is your stored body fat. Total weight = lean mass + fat mass, so if you know one and your weight, you can find the other.
Can I use this calculator to track muscle gain?
It can help spot trends, but the Boer formula only uses height and weight, so it can't tell muscle from other lean tissue. For tracking real muscle change, pair it with a body-fat method like calipers or a DEXA scan and watch the direction over time.
Does lean body mass affect how many calories I burn?
Yes. Lean tissue, especially muscle, is more metabolically active than fat, so people with more lean mass tend to burn more calories at rest. That's why LBM is often used in resting metabolic rate estimates and to guide protein and calorie targets.