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About this tool
Work out lean and fat mass from your body fat percentage, or estimate it with the Boer, James and Hume formulas.
Lean Body Mass Calculator splits your body weight into lean mass and fat mass. Give it a body fat percentage you already know and it does the arithmetic directly; leave that out and it estimates lean mass from height, weight and sex using the Boer, James and Hume equations, showing all three side by side. It also reports fat-free mass index (FFMI), a height-adjusted FFMI and a protein range based on 1.6-2.2 g per kg of lean mass. Handy for lifters, coaches and anyone tracking a cut or a bulk.
Open Lean Body Mass Calculator on AltFTool — it loads instantly in your browser.
Enter the values you already know.
Fine-tune the options to match your scenario.
Read the result and use it in your planning or reporting.
Use a measured body fat percentage for accuracy, or fall back on the classic Boer, James and Hume formulas.
Fat-free mass index and its height-adjusted version show how much muscle you carry for your frame.
Enter kilograms and centimetres or pounds and inches — the conversions happen behind the scenes.
For men, LBM = 0.407 × weight in kg + 0.267 × height in cm − 19.2; for women, LBM = 0.252 × weight + 0.473 × height − 48.3. It is a population estimate, widely used for weight-based drug dosing.
A reliable body fat measurement always wins, because the formulas only see height, weight and sex and cannot tell a muscular person from a heavier one at the same weight.
Roughly 18-20 is typical for untrained men and 15-17 for untrained women; well-trained lifters often reach 22-24, and about 25 is the figure usually cited as the drug-free ceiling. Treat these as rough reference points, not targets.
Sports-nutrition reviews commonly suggest 1.6-2.2 g per kg of lean mass per day when training and dieting, which is the range this tool shows. It is general information — check with a dietitian or doctor about your own needs.
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