Lean Body Mass Calculator

Background.

Lean body mass is everything you weigh that is not stored fat: muscle, bone, organs, blood, skin and the water that fills them. It sits behind several practical decisions. Anaesthetists use it to dose some drugs in people with severe obesity, because the volume those drugs first spread into, and the rate the body clears them, follow lean tissue more closely than total weight. Dietitians and coaches use it when a target is written per kilogram of lean tissue rather than per kilogram on the scale. And it is the other half of every body-fat percentage.

Measuring it takes equipment: a DEXA scanner, an underwater weighing tank, isotope dilution of body water. Estimating it takes a set of scales and a height measurement, and that is what the three equations here do. Hume published the first in 1966, James's appeared in a 1976 UK government report on obesity, and Boer's followed in 1984. Each is a regression: an equation fitted to adults whose lean mass had been measured by a reference method, predicting it from weight, height and sex. The calculator runs all three, shows each one, and reports their average as the headline figure.

With only height and weight to go on, the equations cannot see what the weight is made of. They assume the typical build of the people they were fitted on. A 100 kg, 180 cm rugby forward and a 100 kg, 180 cm office worker get exactly the same answer, so the estimate runs low for very muscular people and can run high for someone who carries a lot of fat on a light frame.

The average also has a weakness at high body weight. The James equation contains a squared weight term that eventually bends its prediction back down: past a body mass index of about 43 in men and 36 in women, each extra kilogram lowers the James estimate, and the average is dragged down with it.

What is lean body mass calculator?

Lean body mass (LBM) is total body weight minus storage fat. In Behnke's classical definition it keeps a small amount of essential lipid, in cell membranes, nervous tissue and bone marrow, and that is what separates it from fat-free mass (FFM), the chemically fat-free remainder. The gap is a few per cent of body weight, smaller than the error of any height-and-weight equation, so in clinical use, and in all three equations here, the terms are interchangeable. Nor is lean mass the same as muscle. Skeletal muscle is its largest single part, but bone, organs, skin and blood count too, so the muscle mass a smart scale reports is a different, smaller number.

Most lean tissue is water. The fat-free body of a healthy adult is close to 73% water, which is why methods that measure body water can be used to estimate lean mass, and why a day of dehydration or fluid retention moves any measurement of it.

The fat mass and body-fat percentage shown with the result are derived, not measured. Fat mass is your weight minus the averaged LBM, and the percentage is that fat mass divided by your weight. Any error in the lean estimate passes straight into both, kilogram for kilogram.

How to use this calculator.

1. 01Weigh yourself in the morning, after the toilet and before eating or drinking, and enter the figure in kilograms or pounds. Pounds are converted at exactly 0.45359237 kg.
2. 02Enter your standing height without shoes, in centimetres or inches. Height enters every equation: a 2 cm error moves the male Boer estimate by about half a kilogram and the female one by nearly a kilogram.
3. 03Choose your sex. Each author fitted separate coefficients for men and women, and at the same height and weight the female equations give less lean mass; at 80 kg and 180 cm the averages differ by 4.31 kg.
4. 04Enter your age. None of the three equations uses it, but all were fitted on adults, so the calculator declines ages under 18.
5. 05Read the average, then the three separate estimates beneath it. If James sits well below the other two, you are probably past its peak, and Boer and Hume are the better guide.

The formula.

All three equations take weight W in kilograms and height H in centimetres. Boer (1984): men 0.407W + 0.267H − 19.2; women 0.252W + 0.473H − 48.3. James (1976): men 1.1W − 128(W/H)²; women 1.07W − 148(W/H)². Hume (1966): men 0.32810W + 0.33929H − 29.5336; women 0.29569W + 0.41813H − 43.2933. The headline figure is the plain mean, (Boer + James + Hume) ÷ 3. Fat mass is W minus that mean, and body-fat percentage is fat mass ÷ W × 100.

Boer and Hume are straight lines. Each extra kilogram of body weight adds a fixed fraction of a kilogram of lean mass, 0.407 kg in Boer's male equation and about 0.33 kg in Hume's, and each centimetre of height adds roughly a quarter to a third of a kilogram. Put another way, the adults these equations were fitted on gained mostly fat, not lean tissue, as they got heavier at a given height.

James behaves differently. Its second term grows with the square of W/H, so it subtracts more and more as weight rises at a fixed height. Setting the slope of the male equation to zero puts its peak at W = 1.1H² ÷ 256, which is a body mass index of about 43 whatever the height; the female equation peaks at a BMI of about 36. Beyond those points the James estimate falls as weight rises, and it reaches zero at a BMI of about 86 in men. The equation was never meant for that range, but the average includes it at every weight.

None of the equations has a term for age, ethnicity, training or hydration. All four change real lean mass, and between them they account for most of the error in any one person's estimate.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Boer P — Estimated lean body mass as an index for normalization of body fluid volumes in humans. American Journal of Physiology 247(4):F632–F636 (1984). The most-cited modern LBM equations, derived in 81 healthy adults using deuterium-dilution total body water as the reference; embedded today in the propofol target-controlled infusion models used in operating rooms.
2. [2]Hume R — Prediction of lean body mass from height and weight. Journal of Clinical Pathology 19(4):389–391 (1966). The first published LBM regression, based on whole-body potassium-40 counting in 95 healthy adults at the Royal Infirmary of Edinburgh.
3. [3]James WPT — Research on Obesity: A Report of the DHSS/MRC Group. Her Majesty's Stationery Office, London (1976). The HMSO Research on Obesity report commissioned by the UK Department of Health and Social Security, including the quadratic weight-to-height LBM equation now known as the James formula.
4. [4]Janmahasatian S, Duffull SB, Ash S, Ward LC, Byrne NM, Green B — Quantification of lean bodyweight. Clinical Pharmacokinetics 44(10):1051–1065 (2005). Modern LBM equation derived specifically for pharmacokinetic dosing in obese patients, now recommended for use in enhanced-recovery anesthesia protocols.
5. [5]Ingrande J, Lemmens HJM — Dose adjustment of anaesthetics in the morbidly obese. British Journal of Anaesthesia 105(Suppl 1):i16–i23 (2010). Sinclair Methodist Hospital and Stanford anesthesia review covering why LBM, not total body weight, is the correct scalar for propofol, rocuronium, and remifentanil dosing in obese adults.
6. [6]Jäger R, Kerksick CM, Campbell BI, et al. — International Society of Sports Nutrition Position Stand: protein and exercise. Journal of the International Society of Sports Nutrition 14:20 (2017). Established the 1.6–2.2 g/kg LBM daily protein target widely used in sports nutrition planning.