Lean Body Mass Calculator
Estimate lean mass and fat mass from height and weight alone, using the Boer, James and Hume formulas.
Your details
These formulas were derived separately for each sex and are not interchangeable.
Optional
If you have a measured figure from a scan or callipers, enter it for a direct calculation. Leave at zero to use the formulas.
This calculator runs entirely on your device. Nothing you enter is uploaded, stored, or sold. How this works
Lean body mass
133.7 lb
average of Boer, James and Hume
Includes your inputs, the full breakdown, and every row of the table.
Fat mass
46.3 lb
Implied body fat
25.7%
FFMI
19.2
Height-adjusted FFMI
19.3
- Body weight
- 180.0 lb
- Boer formula
- 135.6 lb
- James formula
- 138.5 lb
- Hume formula
- 126.9 lb
- Lean body mass used
- 133.7 lb
- Fat mass
- 46.3 lb
What this means
- These formulas infer lean mass from height and weight alone, so they cannot tell a muscular person from a heavier sedentary one of the same size. If you have a DEXA, hydrostatic or calliper reading, entering it above will give a far better answer.
- A height-adjusted FFMI around 19.3 is within the normal natural range — roughly 18 to 20 is average for men, and above 22 indicates substantial training.
How the lean body mass calculation works
Lean body mass is everything you are made of that is not fat: muscle, bone, organs, connective tissue and the water in all of them. It matters clinically because drug clearance, resting metabolic rate and protein requirements all scale with lean tissue rather than total weight, and it matters practically because it is the part of your weight you generally want to keep when losing fat.
The Boer, James and Hume equations all predict lean mass from height and weight, and they diverge in the way you would expect from that limitation — they cannot see body composition, only size. James in particular includes a squared term that causes it to underestimate badly at higher weights, which is a known and documented failure mode. Boer is generally regarded as the most reliable of the three for adults across a normal range.
The fat free mass index shown alongside is lean mass divided by height squared, effectively a BMI that ignores fat. It is the more useful number for anyone training, because it separates the two things BMI conflates. Height-adjusting it makes the figure comparable between people of different statures, and the resulting scale is well documented: roughly 18 to 20 for an untrained man, above 22 for someone with years of serious training, and around 25 as the ceiling most drug-free lifters can reach.
Frequently asked questions
How accurate are these formulas?
For a person of typical build they usually land within a couple of kilograms of a DEXA scan. For very muscular or very obese people they can be off substantially, because they infer composition from size alone. Entering a measured body fat percentage bypasses that problem entirely.
What is FFMI and why does it matter?
Fat free mass index is lean mass divided by height in metres squared. It is BMI with the fat removed, which makes it the honest way to compare muscularity between people of different sizes. Unlike BMI it does not penalise a lean athlete for being heavy.
Why does the James formula give a lower number than the others?
It includes a squared weight-to-height term that grows quickly, so it increasingly subtracts lean mass as weight rises. This is a well-documented weakness at higher body weights and is why Boer is generally preferred in clinical settings.
Can I gain lean mass and lose fat at the same time?
Yes, though it is slow and happens most readily in people who are new to training, returning after a break, or carrying a lot of fat. Adequate protein and resistance training are the two requirements. Experienced lifters near their genetic ceiling will find it very difficult.
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Disclaimer. This calculator is provided for general information and educational purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Results are estimates based on the inputs you provide and the assumptions described above. Confirm any figure with a qualified professional before acting on it.