Lean Body Mass (Boer Formula)

What the Lean Body Mass Calculator Does

Lean body mass (LBM) is your total body weight minus the weight of your stored fat. It includes muscle, bone, organs, skin, and the water held inside them. This calculator estimates your lean body mass from just three inputs: sex, weight, and height, using the Boer formula.

It is useful for anyone who wants a number behind their body composition without a body scan. Lifters and athletes track LBM to confirm that gains come from muscle rather than fat. People in a diet phase use it to check they are losing fat and preserving lean tissue. Clinicians and pharmacists also reference LBM because some medication doses are scaled to lean mass rather than total weight.

How It Works: The Boer Formula

The Boer equation, published in 1984, estimates lean body mass in kilograms from weight in kilograms and height in centimeters. There are separate equations for men and women because, on average, the two sexes carry different proportions of muscle and fat at the same weight and height.

The formulas are:

• Men: LBM = (0.407 x weight in kg) + (0.267 x height in cm) - 19.2
• Women: LBM = (0.252 x weight in kg) + (0.473 x height in cm) - 48.3

Worked Example With Real Numbers

Take a man who weighs 80 kg and stands 180 cm tall. Plug the numbers into the male equation: (0.407 x 80) + (0.267 x 180) - 19.2 = 32.56 + 48.06 - 19.2 = 61.42 kg of lean body mass. His fat mass is therefore about 80 - 61.42 = 18.58 kg, roughly 23 percent body fat.

Now take a woman who weighs 65 kg and is 165 cm tall. Using the female equation: (0.252 x 65) + (0.473 x 165) - 48.3 = 16.38 + 78.05 - 48.3 = 46.13 kg of lean body mass, leaving about 18.87 kg of fat, or roughly 29 percent body fat.

If your figures are in pounds and inches, convert first: divide pounds by 2.2046 to get kilograms, and multiply inches by 2.54 to get centimeters.

Factors That Affect the Result

The Boer formula is a population-based estimate, not a direct measurement. It assumes an average body composition for a given sex, weight, and height, so individuals who fall far from that average will see less accurate numbers.

Keep these influences in mind:

• Muscularity: a heavily trained lifter has more lean mass than the formula predicts, while a sedentary person of the same size may have less.
• Body fat extremes: at very high or very low body fat the equation tends to be least reliable.
• Hydration: temporary water loss or retention changes the scale weight you feed in and shifts the result.
• Sex selection: the two equations differ substantially, so choosing the wrong one produces a large error.

Tips and Common Mistakes

Weigh yourself under consistent conditions, ideally first thing in the morning after using the bathroom and before eating or drinking. Day-to-day swings in food and water can move the scale by a kilogram or more and distort a single reading.

Use the number for trends, not as an exact figure. Recalculating every week or two and watching the direction of change tells you far more than one snapshot. If LBM holds steady while total weight drops, you are losing fat; if LBM falls sharply, you may be losing muscle and should review your protein intake and training.

Common mistakes include mixing units (entering height in inches but weight in kilograms), using the male formula for a female or vice versa, and treating the output as a clinical diagnosis. For body-composition decisions that matter medically, confirm with a DEXA scan or another direct method.