𦴠Lean Body Mass Calculator
Your muscle, bone, and organ mass, estimated three ways.
For informational purposes only, not medical advice. Consult a healthcare professional.
What Lean Body Mass Is
Lean body mass is everything that is not fat: muscle, bone, organs, water, and connective tissue. It is the engine behind your metabolism, and it is the number that matters when dosing certain medications, setting protein targets, and tracking whether a diet is costing you muscle. This calculator estimates it three ways, with the Boer, James, and Hume formulas, and shows the range so you can see the agreement between methods.
The formulas: Boer (men: 0.407 x weight in kg + 0.267 x height in cm - 19.2; women: 0.252 x weight + 0.473 x height - 48.3), James (men: 1.1 x weight - 128 x (weight/height in cm)^2; women: 1.07 x weight - 148 x (weight/height)^2), and Hume (men: 0.32810 x weight + 0.33929 x height - 29.5336; women: 0.29569 x weight + 0.41813 x height - 43.2933). They were developed for pharmacology, where lean mass predicts drug distribution better than total weight.
Why Track It
Scale weight alone cannot tell fat loss from muscle loss. If you lose 10 pounds and your lean mass estimate holds steady, that was nearly all fat: a perfect cut. If lean mass drops 3 pounds, you lost muscle and should raise protein and add resistance training. Protein targets are also set per pound of lean mass, not total weight: 0.7 to 1 gram per pound of lean mass daily is the evidence-based range for active people.
Limits of the Estimate
These formulas assume average body composition for your height and weight. Very lean or very obese individuals fall outside the assumptions, and the James formula in particular misbehaves at extremes. For the most accurate lean mass, a DEXA scan directly measures it. Use these estimates for trends and planning, and recheck monthly under consistent conditions. For informational purposes only, not medical advice. Consult a healthcare professional.
Frequently Asked Questions
What is lean body mass?
Everything except fat: muscle, bone, organs, water, and connective tissue. It drives your metabolism and is the basis for protein targets and some medication dosing.
How accurate are lean mass formulas?
Within a few pounds for average builds. They assume typical body composition, so very lean or very obese people fall outside the assumptions. DEXA scans measure it directly for the highest accuracy.
Why do protein targets use lean mass?
Protein needs scale with muscle, not fat. Setting protein at 0.7 to 1 gram per pound of lean mass avoids overfeeding protein to people with high body fat and underfeeding muscular dieters.
Can I increase lean body mass?
Yes: progressive resistance training plus adequate protein (0.7 to 1 g per pound of lean mass) and a small calorie surplus builds muscle. Beginners can gain 1 to 2 pounds of muscle monthly.
Does losing weight always cost muscle?
Not if you manage it. High protein intake plus strength training during a diet preserves most lean mass. Rapid crash diets without training can cost 25 percent or more of lost weight as muscle.