Lean Body Mass Calculator: Boer, James and Hume

How it works: four lean-mass estimates

This tool compares three historical adult equations with a calculation based on estimated body-fat percentage. Sources are Boer's 1984 paper, Estimated lean body mass as an index for normalization of body fluid volumes in humans; James's 1976 Research on Obesity report; and Hume's 1966 paper, Prediction of lean body mass from height and weight. Each height-weight equation uses its own sex-specific coefficients.

The results are estimates rather than a body-composition examination. The fourth method subtracts estimated fat mass from total body weight. It is commonly used as a fat-free-mass approximation; historical lean-body-mass terminology and measurement models are not perfectly interchangeable. None of these calculations isolates skeletal muscle.

Boer, James, and Hume formulas

In every equation below, W is body weight in kilograms and H is height in centimeters. Convert pounds and inches first. Parentheses matter in James: square the entire W/H ratio, rather than dividing weight by squared height.

The body-fat method is W × (1 − body-fat percentage ÷ 100). Unlike the three historical equations, that arithmetic does not use height or sex coefficients. Changing the selected coefficient group therefore changes three results while leaving the body-fat result unchanged.

Worked example: 80 kg, 180 cm, 20% body fat

Using male coefficients, Boer gives 0.407 × 80 + 0.267 × 180 − 19.2 = 61.42 kg. James gives about 62.72 kg, and Hume gives about 57.79 kg. The body-fat method gives 80 × 0.80 = 64 kg. Those differences are model disagreement, rather than proof of four different tissue measurements.

For the same height and weight with female coefficients, Boer gives 57 kg, James gives about 56.37 kg, and Hume gives about 55.63 kg. The body-fat method still gives 64 kg. This comparison makes the historical assumptions visible; it does not determine which coefficient group accurately represents an individual.

Where the estimates become less useful

The original equations use simple height-weight relationships and binary coefficient groups. They do not directly observe age, training history, hydration, pregnancy, illness, or unusual body proportions. They are not designed to establish a child's body composition. A very muscular or high-body-weight adult can differ substantially from the populations behind a formula.

James includes a negative squared weight-height term, so its behavior can become implausible at high weight relative to height. Above BMI 35, its row shows “Not valid at this BMI” instead of a number. Any historical estimate at or below zero or at or above total body weight is also shown as invalid, while the other results remain available. These checks prevent nonphysical displays; they do not validate the remaining results for clinical use.

Keep body-fat uncertainty visible

At 80 kg, changing the body-fat input from 20% to 25% changes estimated fat-free mass from 64 to 60 kg. The four-kilogram difference follows from the assumption; it does not show that muscle was lost. The Navy body-fat calculator supplies another estimate, with tape-measure limitations of its own.

Use the FFMI calculator only if relating estimated fat-free mass to height serves your goal. For practical next steps, read body recomposition or lean-bulk calories, and use the protein calculator for a separate meal-planning reference. Record actual strength sessions in Trainer so performance, recovery, and consistent measurements can inform progression together.