Lean Body Mass Calculator

What lean body mass is and how it is estimated

Lean body mass (LBM) is your total weight minus the weight of your body fat: muscle, bone, organs, skin, blood and body water. Research papers often call it fat-free mass; the terms are close enough to be used interchangeably here.

Measuring it properly takes equipment such as a DXA scan or tests of total body water. The formulas on this page were fitted to groups of people who were measured, so they predict lean mass from sex, height and weight alone. One common clinical use is scaling doses, for example of CT contrast agents, to lean mass rather than total weight.

How this differs from the body fat calculator. The body fat calculator uses the U.S. Navy tape method: neck, waist and (for women) hip measurements, which respond to where you actually carry fat. This calculator needs no tape measure, but because it only knows height and weight it gives the same answer for a muscular person and a less muscular one of the same size. If you already know your body fat percentage — from that calculator or a scan — enter it and the calculator uses it directly.

The formulas

All three take weight W in kilograms and height H in centimeters and give lean body mass in kilograms. Each has separate coefficients for men and women.

Boer (1984), published in a study of body fluid volumes in healthy adults, is the main result:

James (1976), from a UK government report on obesity research, uses weight relative to height squared:

Hume (1966) was fitted to 29 men and 27 women whose lean mass was worked out from total body water:

If you enter a body fat percentage, lean mass comes straight from it instead:

W

weight in kilograms

H

height in centimeters

BF%

body fat percentage

In every case, fat mass is weight minus lean mass, and lean percentage is lean mass divided by weight.

Why the James formula fails at high BMI. Its squared term grows faster than its linear term. The predicted lean mass therefore peaks and then falls as weight keeps rising. Setting its slope to zero puts the peak at a BMI of 10,000 × 1.10 ÷ 256 ≈ 43.0 for men and 10,000 × 1.07 ÷ 296 ≈ 36.1 for women, which matches the plateau reported in studies of contrast dosing in people with obesity. Above that BMI the calculator still shows the James figure but marks it as not reliable. Boer and Hume are straight lines in weight, so they don’t have this problem; that is why Boer is the headline result.

Worked example

A man who is 5 ft 10 in tall and weighs 180 lb:

1. Height: 70 in × 2.54 = 177.8 cm. Weight: 180 lb × 0.45359237 = 81.65 kg.
2. Boer: 0.407 × 81.65 + 0.267 × 177.8 − 19.2 = 33.23 + 47.47 − 19.2 = 61.50 kg (135.6 lb). Fat mass is 81.65 − 61.50 = 20.14 kg (44.4 lb), so he is 75.3% lean.
3. James: 1.10 × 81.65 − 128 × (81.65 ÷ 177.8)² = 89.81 − 26.99 = 62.82 kg (138.5 lb).
4. Hume: 0.32810 × 81.65 + 0.33929 × 177.8 − 29.5336 = 57.58 kg (126.9 lb).
5. If a scan put his body fat at 20%: 81.65 × 0.80 = 65.32 kg, which is exactly 144 lb.

The three formulas span about 5 kg (11 lb) for the same person, which is a fair picture of their precision.

To see the James problem, take a woman 165 cm tall. At 100 kg (BMI 36.7) James gives 52.6 kg of lean mass; at 120 kg (BMI 44.1) it gives only 50.1 kg — less lean mass for 20 kg more weight. Boer gives 60.0 kg at 120 kg.

Limits of height-and-weight formulas

• They can’t see muscle. Two people of the same sex, height and weight get the same answer even if one is far more muscular. Athletes are usually underestimated.
• Small, older study groups. Hume’s formula came from 56 people; none of the three was built on a large, varied modern sample. Puri and Blake (2022) compared ten such equations against DXA scans in older patients and found they differ in accuracy.
• High BMI. Beyond the peak described above, the James formula is not meaningful. The others keep working mathematically but were not designed for severe obesity.
• Adults only. None of the formulas is meant for children or during pregnancy.