How to Calculate Lean Body Mass: Formulas with Example

Learning how to calculate lean body mass is the quickest way to see what your scale weight really means. The number on the scale mixes fat and lean tissue together, so two people who weigh the same can look and perform very differently. Lean body mass, often shortened to LBM, is everything in your body except fat: muscle, bone, organs, skin and water. Once you know it, you can judge whether a change in weight came from fat, muscle or both, and set protein and training goals that actually match your body. The free Lean Body Mass Calculator does the arithmetic for you, but this guide explains the formula behind it so you can check any result by hand.

Think of your total weight as two piles. One pile is fat mass, and the other is lean body mass. Add them together and you get back to your full body weight, which is a useful check on any calculation. If your weight stays the same but your lean mass rises, you have probably gained muscle and lost fat at the same time, even though the scale did not move. That is why coaches and nutritionists track lean mass over months rather than celebrating or worrying about a single weigh-in. A falling lean mass during a diet, on the other hand, is a warning sign that protein or strength training may need attention.

The Method / Formula

The most widely used height-and-weight method is the Boer formula. It needs only your sex, your weight in kilograms and your height in centimetres, and it gives separate equations for men and women because body composition differs between the sexes on average.

For men: LBM = 0.407 × weight + 0.267 × height − 19.2. For women: LBM = 0.252 × weight + 0.473 × height − 48.3. In both equations, weight is in kilograms and height is in centimetres, and the result is in kilograms. Once you have LBM, fat mass is simply what is left over: fat mass = weight − LBM. You can also turn that into a body fat percentage by dividing fat mass by total weight and multiplying by 100.

• Weight: your total body weight in kilograms. This is the starting point for the whole calculation.
• Height: your height in centimetres. Height matters because taller people tend to carry more lean tissue at the same weight.
• The decimal numbers: these are coefficients fitted from research data. They weight each input by how strongly it predicts lean mass.
• The final number subtracted (19.2 for men, 48.3 for women): a constant that calibrates the equation so its results line up with measured body composition.
• Fat mass: total weight minus LBM. If this step gives a negative number, an input has almost certainly been entered wrong.

Boer is popular because, among formulas that use only height and weight, it matches DEXA scan results most closely for normal and obese adults, better than the older James and Hume formulas. That does not make it a scan. It is still an estimate built from population averages, so very muscular athletes and children may get less accurate results. Treat the number as a consistent tracking tool rather than a medical diagnosis, and compare new results with old ones measured the same way.

How to Calculate Lean Body Mass Step by Step

1. Choose your sex, because the Boer formula uses a different equation for men and women.
2. Weigh yourself in kilograms, ideally first thing in the morning after using the bathroom, wearing light clothing.
3. Measure your height in centimetres, standing straight against a wall without shoes.
4. Enter your sex, weight and height into the calculator, or work through the formula by hand.
5. Read the two results: lean body mass and fat mass, both in kilograms.
6. Check that lean mass plus fat mass equals your total weight, then repeat the measurement every few weeks under the same conditions to track the trend.

Worked Example

Take the example from the tool page: a man who weighs 80 kg and is 180 cm tall. Using the men’s Boer formula, the working is as follows.

• 0.407 × 80 = 32.56
• 0.267 × 180 = 48.06
• 32.56 + 48.06 = 80.62, then 80.62 − 19.2 = 61.42 kg of lean body mass
• Fat mass = 80 − 61.42 = 18.58 kg
• Body fat percentage = 18.58 ÷ 80 × 100 = 23.2 percent

So about 61.42 kg of the 80 kg total is lean tissue, and the remaining 18.58 kg is fat. As a practical follow-up, the common sports-nutrition rule of about 2.2 g of protein per kilogram of lean body mass gives roughly 135 g of protein per day for this person (2.2 × 61.42 = 135.1 g), a useful starting point rather than a strict prescription.

Common Mistakes / Good to Know

• Using the wrong equation: the men’s and women’s formulas give different answers for the same weight and height, so always pick the one that matches the person being measured.
• Mixing up units: the Boer constants assume kilograms and centimetres. Entering pounds, inches or height in metres will give a meaningless result, so convert first.
• Calling lean mass muscle mass: lean body mass includes bones, organs, skin and water as well as muscle, so a gain in lean mass is not all muscle gain.
• Treating the estimate as a scan: this is an estimate, not a DEXA result. It is best used to follow trends over time, measured under consistent conditions, and it is not a medical diagnosis.

For an instant check, try the Body Weight Calculator, and read our related guide How to Calculate Ideal Body Weight: Devine Formula Explained to see how a target weight compares with your lean mass.

Frequently Asked Questions