How to use it
- Choose your Sex, then enter your Weight in kilograms and your Height in centimetres.
- Enter your Body fat (optional) if you know it, or leave it empty.
- Read Lean body mass, range across the estimates, then compare the estimates in the table.
How the estimates are calculated
Lean body mass (LBM) is the body weight without body fat. It has been described as a better guide than total weight for some medicine doses, because body fat is less relevant for metabolism (Wikipedia, Lean body mass).
Boer (1984) and Hume (1966) estimate lean mass from weight and height with separate equations for men and women. The James equation (1976), as written by Caruso and colleagues (2018), uses weight and the square of its ratio to height. The body fat method follows from the definition: lean mass is weight multiplied by one minus the body fat fraction.
These are estimates for information only, not medical advice. Any dose that uses lean body mass is set by the prescriber.
Boer, men: LBM = 0.407 x W + 0.267 x H - 19.2
Boer, women: LBM = 0.252 x W + 0.473 x H - 48.3
Hume, men: LBM = 0.32810 x W + 0.33929 x H - 29.5336
Hume, women: LBM = 0.29569 x W + 0.41813 x H - 43.2933
James, men: LBM = 1.10 x W - 128 x (W / H)^2
James, women: LBM = 1.07 x W - 148 x (W / H)^2
Body fat: LBM = W x (1 - body fat / 100)
W is weight in kg, H is height in cm.Which estimate should I use?
The equations differ because each was fitted to a different group of people. The table shows each one, so you can see how far they differ for your size. If you know your body fat percentage from a reliable measurement, the body fat row is the most direct figure, but it is only as good as that measurement.
Ask your doctor or pharmacist which estimate their protocol uses. A clinic that doses by lean mass chooses one equation and keeps to it.
What this calculator does not include
- A measured lean mass from a scan such as DXA, which is more direct than any formula.
- Pregnancy, which adds fluid and tissue that these equations do not model.
- Illness, fluid shifts and medicines, which can change a measured lean mass from day to day.
Worked example: woman, 165 cm, 65 kg, 25 percent body fat
Boer: 0.252 x 65 + 0.473 x 165 - 48.3 = 46.1 kg.
James: 1.07 x 65 - 148 x (65 / 165)^2 = 46.6 kg.
Hume: 0.29569 x 65 + 0.41813 x 165 - 43.2933 = 44.9 kg.
Body fat: 65 x (1 - 0.25) = 48.8 kg, so the fat mass is 16.3 kg. The lean body mass range is 44.9 to 48.8 kg.
| Estimate | Lean body mass | Fat mass |
|---|---|---|
| Boer (1984) | 46.1 kg | 18.9 kg |
| James (1976) | 46.6 kg | 18.4 kg |
| Hume (1966) | 44.9 kg | 20.1 kg |
| From body fat percentage | 48.8 kg | 16.3 kg |
Frequently asked questions
What is the difference between lean body mass and fat-free mass?
Lean body mass is body weight minus body fat. Fat-free mass is very close to it, and the difference is that lean body mass counts cell membranes as lean, which is only a small part: up to 3 percent of body mass in men and 5 percent in women (Wikipedia, Lean body mass).
Which formula is the most accurate?
None is exact for everyone. The equations were fitted to different groups, so they give different answers for the same person. A measured value from a DXA scan or a similar test is more direct.
Why does the page show four answers?
Because the estimates differ. The table shows the spread, and the body fat row uses the percentage you entered, so you can see which answer depends on your own measurement.
Why is lean body mass used for some medicine doses?
Some anaesthetic and contrast doses are based on lean body weight rather than total weight, because body fat is less relevant for metabolism (Wikipedia, Lean body mass). The team treating you sets any dose.
Does this calculator store the numbers I enter?
No. The calculation runs in your browser and nothing you enter is sent to us.
Sources
- Boer P (1984): Estimated lean body mass as an index for normalization of body fluid volumes in humans, Am J Physiol (PubMed)
- Hume R (1966): Prediction of lean body mass from height and weight, J Clin Pathol 19:389-391 (PubMed)
- Caruso D et al. (2018): Lean body weight-tailored iodinated contrast injection in obese patient: Boer versus James formula, BioMed Res Int (PubMed; gives the James equation)
- Lean body mass, with the Boer and Hume equations (Wikipedia)
Figures and text last checked on Tuesday, November 10, 2026.