Lean Body Mass Calculator

Lean Body Mass (LBM) is your total body weight minus all fat mass. It includes muscle, bone, organs, blood, and water.

Fat Mass = Weight × (Body Fat % / 100) Lean Body Mass = Weight − Fat Mass

The arithmetic is trivial. Everything that makes this number good or useless happens upstream, in how you got the body fat percentage.

Three ways to get there, and this calculator offers all of them:

1. You already know your body fat percentage. Enter it directly. Best if it came from a DEXA scan or calipers in competent hands.

2. The U.S. Navy tape method. Needs a tape measure and nothing else.

Men: Body Fat % = 86.010 × log₁₀(Waist − Neck) − 70.041 × log₁₀(Height) + 36.76 Women: Body Fat % = 163.205 × log₁₀(Waist + Hip − Neck) − 97.684 × log₁₀(Height) − 78.387

The formula is written for inches, which is why those coefficients look the way they do. Enter your measurements in whichever unit you picked above and the calculator converts. Measure the waist at the navel, the neck just below the larynx, and the hips at the widest point, and keep the tape snug without compressing.

3. The Boer formula, which skips body fat entirely and estimates LBM from height and weight:

Men: LBM (kg) = 0.407 × weight(kg) + 0.267 × height(cm) − 19.2 Women: LBM (kg) = 0.252 × weight(kg) + 0.473 × height(cm) − 48.3

Boer is what clinicians reach for when there is no body composition data, mostly for drug dosing. It cannot tell a lifter from a sedentary person of the same size, so treat it as a population average rather than a measurement of you.

How accurate is each method?

Typical body fat ranges:

Lean body mass is not muscle mass. This is the mistake almost everyone makes with the number. LBM counts bone, organs, blood and water alongside muscle. Skeletal muscle alone is usually around 40-45% of body weight in men and 30-35% in women, while LBM typically lands at 75-90%. For a 200 lb man at 20% body fat, LBM is 160 lb but actual muscle is closer to 85 lb.

LBM and fat-free mass are not quite the same either. The terms get swapped freely. Fat-free mass excludes every gram of fat. Lean body mass includes the essential fat your organs and hormones need, roughly 3% in men and 12% in women. The gap is small, but in a clinical setting where a dose depends on it, small matters.

Lean mass declines with age. Without resistance training, adults lose roughly 3-8% of muscle per decade after 30, and the rate picks up after 60. Two people at the same height and weight can have very different LBM if one is 25 and the other 70. That is why LBM-based dosing beats weight-based dosing in elderly patients, and why lifting matters more with age, not less.

Track the trend, not the reading. Measurement error on any of these methods is larger than a month of honest progress. If LBM is climbing while the scale holds steady, you are gaining muscle and losing fat at the same time, which is the outcome everyone wants and almost nobody notices without tracking.

Unit conversions: 1 kg = 2.205 lbs | 1 cm = 0.394 in