Lean body mass calculator
Everything you weigh except fat, estimated from your height, weight and sex by three published equations: Boer (1984), James (1976, in a UK government report on obesity research) and Hume (1966). Hume fitted his on 56 adults whose lean mass was calculated from measured body water. Give a body fat figure and a fourth line works it out from that instead.
What the number actually means
Your body splits into two parts for this purpose: fat mass, and everything else. That everything else is your lean body mass, and it includes muscle, bone, organs, skin and the water held in all of them. People often read it as a muscle figure. It is not. Muscle is one part of lean mass, and none of these equations can tell you how much of it you have.
The three equations
All three take height, weight and sex, and nothing else. They were fitted decades apart on different groups of people, which is why they return different answers for you.
| Equation | Published | Why it exists |
|---|---|---|
| Boer | 1984 | Derived to normalise body fluid volumes between people. The one most often used in clinical practice, and the one shown first here for that reason. |
| James | 1976 | Came out of a UK government obesity research group. Uses weight against height squared, so its answer bends at the heavier end. |
| Hume | 1966 | The oldest of the three, and usually the lowest estimate. Still quoted, mostly for continuity with older clinical work. |
The better route, if you can take it
A prediction from height and weight cannot see your build. Subtracting a measured body fat percentage from your weight can, because that percentage came from your actual measurements. If you have a tape measure, spend the minute: estimate body fat first, and the lean mass figure that follows responds to you rather than to a population average.
There is one more step worth knowing about. Lean mass rises with body size, so comparing yours against someone taller tells you very little. Thefat free mass index divides lean mass by height squared and adjusts to a common reference height, which makes that comparison mean something.
There is no healthy range for this number
BMI has cut points. Body fat has cut points. Lean body mass has neither, because no health body publishes one. It is a component of your weight, not an assessment of it, and a larger figure mostly reflects a larger person. Use it as a baseline you can watch over months, not as a score to beat.
Common questions
What is lean body mass?
Everything in your body that is not fat: muscle, bone, organs, skin and the water in all of them. It is not the same as muscle mass, which is only one part of it, and a calculator that tells you your lean body mass has not told you how much muscle you carry.
Which lean body mass formula is best?
None of the three predicts your lean mass from height and weight better than simply subtracting a measured body fat percentage from your weight. If you have a body fat estimate, use that route. Among the three prediction equations, Boer is the one most commonly used in clinical practice, and it is the one this page leads with for that reason rather than because it is more correct for you.
Why do the three formulas disagree?
They were built from different populations for different purposes across three decades: Hume in 1966, James in 1976 and Boer in 1984. Each one fitted coefficients to the people in front of it, and those groups differed in build and in body composition. The gap between the answers is a fair picture of how much confidence a height and weight prediction deserves.
Can these formulas tell muscle from fat?
No, and this is their real limit. They take only your height, your weight and your sex, so two people of identical height and weight get an identical answer whether one of them trains daily and the other does not. If you want a number that responds to your actual body composition, estimate body fat from a tape measure first and subtract it.
What is lean body mass used for?
In clinical settings it is used to scale things that track lean tissue rather than total weight, including some drug doses and fluid volume estimates. That is what the Boer and James equations were derived for. Outside that, it is mostly used as a reference point for tracking your own change over time.
Is a higher lean body mass always better?
No. It is a measurement, not a target, and there is no published healthy range for lean body mass the way there is for BMI or body fat. It rises with total body size, so a heavier person usually has more lean mass as well as more fat mass. If you want a figure that adjusts lean mass for your height, the fat free mass index does that.
Sources
Every figure above comes from one of these. Check them.
- Boer P (1984). Estimated lean body mass as an index for normalization of body fluid volumes in humans. American Journal of Physiology 247(4 Pt 2), F632-F636
- James WPT (1976). Research on Obesity: a report of the DHSS/MRC group. London, Her Majesty's Stationery Office
- Hume R (1966). Prediction of lean body mass from height and weight. Journal of Clinical Pathology 19(4), 389-391
- Kouri EM, Pope HG, Katz DL, Oliva P (1995). Fat-free mass index in users and nonusers of anabolic-androgenic steroids. Clinical Journal of Sport Medicine 5(4), 223-228
A baseline is only useful if you keep it
The app stores each set of measurements, so the lean mass figure has something to be compared against next time.
Free, no account, and it works offline. 100,000+ downloads on Google Play.
This is not medical advice. Body Numbers is a calculator. It does not diagnose, treat or monitor any condition, and no number it produces should replace a conversation with a doctor or a dietitian. If a result worries you, or if you are pregnant, unwell, or under eighteen, speak to a health professional.
