Lean Body Mass Calculator

How to Calculate Lean Body Mass

Lean body mass (LBM) is everything in your body that isn't fat — muscle, bone, organs, and water. This calculator uses the Boer formula, published by Peter Boer in 1984 and one of the most widely used formula-based methods for estimating LBM, since it only requires weight, height, and gender rather than specialized equipment.

$$\text{Men: LBM} = 0.407w + 0.267h - 19.2 \qquad \text{Women: LBM} = 0.252w + 0.473h - 48.3$$

Once LBM is known, fat mass is simply total weight minus LBM, and body fat percentage is fat mass expressed as a share of total weight.

Weight and height can both be entered in imperial (pounds, feet/inches) or metric (kilograms, centimeters) using the Unit System selector above — the calculator converts internally to kilograms and centimeters before running the Boer formula, so switching units doesn't change the underlying result.

Worked Example: A 180-Pound, 5'10" Male

180 pounds converts to about 81.6 kg, and 5'10" converts to about 177.8 cm. Plugging into the men's formula: $\text{LBM} = 0.407 \times 81.6 + 0.267 \times 177.8 - 19.2 = 33.2 + 47.5 - 19.2 = 61.5$ kg. Fat mass is $81.6 - 61.5 = 20.1$ kg, and estimated body fat percentage is $20.1 / 81.6 \times 100 \approx 24.6\%$.

Lean Body Mass vs. Body Weight — Why the Distinction Matters

Total body weight alone doesn't distinguish between muscle and fat, so two people at the identical weight and height can have very different lean body mass depending on their body composition. Tracking lean body mass over time — rather than weight alone — gives a clearer picture during a training program, since the goal for many lifters is gaining lean mass while keeping fat mass flat or even reducing it, a distinction a bathroom scale alone can't show.

Why Formula-Based Estimates Have Limits

The Boer formula, like other weight-and-height-based body composition formulas, was derived from regression analysis against a reference population and doesn't directly measure any individual's actual tissue composition. It tends to be reasonably accurate for people close to the average build of that reference population, but less precise for people with unusually high muscle mass (where it can underestimate LBM) or unusually high body fat (where it can overestimate LBM). Direct measurement methods like DEXA scans or hydrostatic weighing are more accurate but require specialized equipment not available to most people at home.

Common Lean Body Mass Calculation Mistakes

Mixing up the men's and women's coefficient sets is an easy manual-calculation error, since both formulas share the same overall structure but use different multipliers for weight, height, and the constant term. Forgetting to convert weight and height into the units the formula expects — kilograms and centimeters — before plugging in the numbers is another common mistake when calculating this by hand rather than using a converting calculator. Treating the resulting body fat percentage as lab-precise, rather than a formula-based estimate with real margin of error, can also lead to over-interpreting small changes between two calculations taken close together in time.

Lean Body Mass Terms You Should Know

Lean Body Mass (LBM) — the weight of everything in the body that isn't fat: muscle, bone, organs, and water.

Fat Mass — the weight of body fat specifically, calculated here as total weight minus lean body mass.

Boer Formula — a 1984 formula estimating LBM from weight, height, and gender, one of several competing formula-based LBM estimation methods.

DEXA Scan — a specialized X-ray-based method considered a clinical gold standard for measuring body composition directly, far more precise than formula-based estimates like this one.