Lean Body Mass Calculator

Background.

This lean body mass calculator estimates your lean body mass (LBM) — the part of your body weight that is not adipose tissue — by averaging three of the most cited regression equations in clinical medicine: Boer (1984), James (1976), and Hume (1966). You enter weight, height, sex, and age; the tool returns the averaged LBM in kilograms and pounds, your fat mass, your body fat percentage, and each of the three individual formula estimates side by side. Nothing leaves your browser; there is no submit button and no account requirement. The reason this number matters far beyond fitness apps is that LBM, not total body weight, is the correct dosing scalar for several anesthetic and intensive-care drugs. Propofol's volume of distribution at steady state, for instance, scales with lean body mass rather than with total body weight, which is why dosing an obese surgical patient on total body weight at induction has a well-documented risk of severe hypotension and prolonged emergence. Janmahasatian and colleagues showed this rigorously in a 2005 Clinical Pharmacokinetics paper that has since shaped enhanced-recovery anesthesia protocols across major hospital systems. Outside the operating theatre, oncologists use LBM to dose chemotherapeutic agents like carboplatin and cisplatin where lean tissue better predicts drug clearance than body surface area, and sports scientists use it as the denominator for protein-intake targets (the 1.6–2.2 g/kg LBM range cited by the International Society of Sports Nutrition assumes you know your lean mass, not just your scale weight).

A common point of confusion worth clearing up at the outset: lean body mass and fat-free mass (FFM) are not strictly the same thing in the strict biochemistry literature, although the three formulas here treat them as interchangeable. FFM is everything except lipids of any kind, including the small but real mass of essential intramembrane and bone-marrow lipids that are part of normal cell structure. LBM, in the classical Behnke definition, includes those essential lipids and excludes only storage adipose tissue. The numerical difference is on the order of 2–3% of body weight in lean adults, which is below the standard error of the regression equations themselves, so for any practical purpose — drug dosing, nutrition planning, body composition tracking — clinicians and dietitians use LBM and FFM as synonyms. We do the same here.

The three formulas this calculator averages were derived in different decades, in different populations, with different reference methods, and they disagree by several kilograms on the same person. Boer, working with deuterium-dilution total body water measurements in 81 healthy adults, published the most-cited modern equations in the American Journal of Physiology in 1984: LBM (male) = 0.407·W + 0.267·H − 19.2 and LBM (female) = 0.252·W + 0.473·H − 48.3, with weight in kilograms and height in centimetres. Boer's equations are the ones overwhelmingly cited in the modern anesthesia pharmacology literature, including the propofol pharmacokinetic models implemented in target-controlled infusion pumps used in operating rooms today. James published his equations in 1976 as part of the HMSO Research on Obesity report commissioned by the UK Department of Health and Social Security: LBM (male) = 1.1·W − 128·(W/H)² and LBM (female) = 1.07·W − 148·(W/H)². The James equations are unusual in that they use a quadratic weight-to-height term, which makes them more sensitive to height in proportion to weight than a linear regression would be, and they tend to undershoot Boer for tall, lean people. Hume, working at the Royal Infirmary of Edinburgh in 1966, produced the first published formula in the Journal of Clinical Pathology and based it on whole-body potassium-40 counting in 95 healthy adults: LBM (male) = 0.32810·W + 0.33929·H − 29.5336 and LBM (female) = 0.29569·W + 0.41813·H − 43.2933. Hume's equation systematically returns the lowest LBM of the three for typical adult proportions, partly because potassium-40 counting captures intracellular potassium pools that differ from the total-body-water reference Boer used.

Why average the three rather than pick one? Because any single regression carries the bias of the specific population in which it was derived — Boer in 81 adults at a single Dutch hospital, Hume in 95 patients in Edinburgh, James in a UK reference population — and on any individual outside the derivation cohort the three estimates can disagree by 3–5 kg without anyone being wrong. The mean of three independent estimates is, on average, closer to a reference DEXA or hydrostatic-weighing measurement than any single estimate, and reporting the three side by side gives you an honest read on the spread. If Boer says 62 kg, James says 63 kg, and Hume says 58 kg, the answer is 'somewhere between 58 and 63', and the 60.6 kg mean is a defensible central estimate to act on for drug dosing or protein-intake planning.

The calculator also reports fat mass (body weight minus averaged LBM) and body fat percentage. Treat the body fat percentage as a coarse estimate — if you need a clinically reliable absolute number, DEXA is the practical reference standard today, with hydrostatic weighing and air-displacement plethysmography (BodPod) as the legacy gold-standard methods. Regression-based formulas like the three averaged here have a typical standard error of 3–5 percentage points of body fat against DEXA, which means they are excellent for tracking changes over time at the same scale and tape measure but unreliable for snapshot comparisons against published athlete reference ranges. With those caveats stated, the averaged LBM number on this page is the single most useful body-composition output you can derive from weight and height alone, and it is the input most clinical pharmacology guidelines now recommend for weight-based drug dosing in obese adults.

What is lean body mass calculator?

Lean body mass (LBM) is the portion of total body weight that is not storage adipose tissue. In the classical Behnke definition it comprises muscle, bone, organs, connective tissue, body water, and a small fraction of essential intramembrane and bone-marrow lipids; everything else (storage subcutaneous and visceral fat) is excluded. In modern clinical practice the terms 'lean body mass' and 'fat-free mass' (FFM) are used interchangeably, although strict biochemistry reserves FFM for the truly lipid-free fraction. LBM is used as a drug-dosing scalar in anesthesia and oncology — propofol's volume of distribution at steady state scales with LBM, not total body weight (Janmahasatian et al., 2005, Clinical Pharmacokinetics) — and as the denominator for protein-intake targets in sports nutrition (1.6–2.2 g/kg LBM, ISSN 2017 position stand). Because LBM cannot be measured directly with a tape measure, clinicians estimate it from weight, height, sex, and age using regression equations. The three most cited regressions — Boer (1984), James (1976), and Hume (1966) — were each derived in a different reference population using a different gold-standard method (Boer used total-body-water dilution, Hume used potassium-40 counting, James used a mixed reference), and they disagree by 3–5 kg on the same individual. The most defensible point estimate of LBM from weight and height alone is therefore the mean of the three formulas, which is what this calculator returns as its primary output.

How to use this calculator.

1. 01Enter your current body weight. Use a recent morning measurement after using the bathroom and before eating for the cleanest reading.
2. 02Choose your weight unit (kg or lb). Pounds are converted to kilograms internally using the NIST SP 811 exact factor 1 lb = 0.45359237 kg.
3. 03Enter your standing height without shoes.
4. 04Choose your height unit (cm or in). Inches are converted to centimetres internally using the NIST SP 811 exact factor 1 in = 2.54 cm.
5. 05Select your biological sex. Boer, James, and Hume each publish separate regression coefficients for males and females because lean tissue distribution differs by sex.
6. 06Enter your age in years. Age is used for input validation and is displayed alongside the result; the three formulas in this calculator are not themselves age-adjusted.
7. 07Read the primary Lean Body Mass value in kilograms — this is the average of the three formulas — and the same value converted to pounds.
8. 08Read the three individual formula estimates side by side. A spread of 3–5 kg between Boer, James, and Hume is normal and reflects the populations each regression was derived in.
9. 09Read the derived fat mass (body weight − averaged LBM) and body fat percentage. Treat the body fat percentage as a coarse estimate — DEXA, hydrostatic weighing, and BodPod give more reliable absolute values.

The formula.

All three formulas take weight (W) in kilograms and height (H) in centimetres. Boer (1984): for males LBM = 0.407·W + 0.267·H − 19.2; for females LBM = 0.252·W + 0.473·H − 48.3. Boer derived these regressions in 81 healthy adults using deuterium-dilution total body water as the reference; they are the most-cited LBM equations in modern anesthesia pharmacology and are embedded in the propofol target-controlled infusion models used in operating rooms today. James (1976): for males LBM = 1.1·W − 128·(W/H)²; for females LBM = 1.07·W − 148·(W/H)². The quadratic weight-to-height term makes James more sensitive to body proportions than a linear regression and the equation tends to undershoot Boer for tall, lean people. James published these in the HMSO Research on Obesity report commissioned by the UK Department of Health and Social Security. Hume (1966): for males LBM = 0.32810·W + 0.33929·H − 29.5336; for females LBM = 0.29569·W + 0.41813·H − 43.2933. Hume's equation was the first published LBM regression and was based on whole-body potassium-40 counting in 95 adults at the Royal Infirmary of Edinburgh; it systematically returns the lowest LBM of the three for typical adult proportions. The primary calculator output is LBM = (Boer + James + Hume) ÷ 3, which reduces the bias of any single derivation cohort. Fat mass is computed as total body weight minus the averaged LBM, and body fat percentage as fat mass ÷ body weight × 100. All internal arithmetic is performed in Decimal.js at full precision; unit conversions use the NIST SP 811 exact factors 1 lb = 0.45359237 kg and 1 in = 2.54 cm.

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.