Lean Body Mass Calculator
This lean body mass calculator estimates how much of your weight is not fat, using three published height-and-weight formulas: Boer, James and Hume. If you already know your body fat percentage from a scan, scale or tape method, add it to get a fourth, more direct figure. A table lines the results up side by side so you can see how much they differ.
Updated
Results
- Lean body mass (James)Unreliable at high BMI
- Lean body mass (Hume)
- Lean body mass (from body fat %)Shown when you enter body fat
- Implied body fat (Boer)
- Implied fat mass (Boer)
- Note
| Method | Lean body mass | % of body weight | Body fat |
|---|
Results are estimates. See the methodology below.
How to use the calculator
- Choose US (ft, in, lb) or metric (cm, kg) units.
- Select your sex. The formulas use different coefficients for men and women.
- Enter your height and weight, ideally measured without shoes and in light clothing.
- Optionally, enter your body fat percentage. The body-fat-based result only appears when you fill in this field.
- Read the Boer estimate (the main result), the James and Hume estimates, and the body fat and fat mass implied by Boer.
Formulas used
All three equations work in kilograms (W) and centimeters (H). In US units the calculator converts your inputs, runs the formula and shows the result in pounds.
| Formula | Men | Women |
|---|---|---|
| Boer (1984) | 0.407W + 0.267H − 19.2 | 0.252W + 0.473H − 48.3 |
| James (1976) | 1.1W − 128(W/H)² | 1.07W − 148(W/H)² |
| Hume (1966) | 0.32810W + 0.33929H − 29.5336 | 0.29569W + 0.41813H − 43.2933 |
With a known body fat percentage, lean body mass is simply weight × (1 − body fat %). The implied body fat shown for Boer is 1 − Boer LBM ÷ weight.
The James equation is the original 1976 version with 128 for men; a later adaptation by Sugawara and colleagues uses 120 instead, which gives slightly different numbers.
Why James can go wrong at higher weights
Because James subtracts a term that grows with the square of weight, its result rises, peaks and then falls as weight keeps increasing. The peak sits at a BMI of about 43 for men and 36 for women. Above that point the calculator shows a warning, and the James figure should not be used.
Examples
Man, 5 ft 10 in, 176 lb
Boer gives 134.0 lb of lean body mass, James 136.7 lb and Hume 125.6 lb, about 11 lb between the highest and lowest. Boer implies 23.9% body fat, or 42.0 lb of fat mass. If a scan had measured him at 18% body fat, his lean body mass would be 144.3 lb, higher than all three formulas.
Woman, 5 ft 5 in, 140 lb
The formulas agree more closely here: Boer 101.0 lb, James 101.5 lb and Hume 98.1 lb. Boer implies 27.9% body fat.
Woman, 5 ft 3 in, 265 lb
Boer gives 127.2 lb and Hume 130.4 lb, but James drops to 99.4 lb because her weight is past the formula’s peak. The calculator flags this, and the James result should be ignored.
How to read the result
Lean body mass is body weight minus body fat, so it includes organs, bone and water as well as muscle. It is not the same as muscle mass, and a higher number does not by itself mean you are more muscular.
The height-and-weight formulas cannot see your actual body composition. Two people with the same height and weight get the same estimate, even if one is a lifter and the other is sedentary. When you have a measured body fat percentage, the body-fat-based figure is usually more informative than the formulas. For a direct measurement, ask a clinician about methods such as DEXA.
Frequently asked questions
What is lean body mass?
Lean body mass (LBM) is your total weight minus your body fat. It covers muscle, bone, organs, skin and body water.
Which lean body mass formula is best?
Each has a purpose. Boer is the main result here and is often preferred, for example, for working out CT contrast doses in people with obesity. Hume is the oldest of the three, and James breaks down at higher BMI. If you know your body fat percentage, the body-fat-based result reflects your own composition.
Why do the formulas give different numbers?
They come from different studies of different people, each fitted to its own sample, so none matches every body type. In the first example above, they differ by about 11 lb for the same man.
Is lean body mass the same as fat-free mass?
Almost. The terms are often used interchangeably, but lean body mass also counts the small amount of fat in cell membranes, so it is slightly higher than strict fat-free mass.
What is lean body mass used for?
Besides fitness tracking, clinicians sometimes use lean body weight to dose certain medicines and CT contrast, especially in people with obesity. Those decisions belong to your care team, not to an online calculator.
Can I use this calculator for children?
No. The Boer, James and Hume equations were developed for adults. A pediatrician can assess a child’s growth and body composition.
Sources
This calculator gives formula-based estimates for adults and is not a body composition measurement or medical advice. Medication or contrast dosing must be decided by a qualified clinician.