Lean body mass calculator

Everything you're made of that isn't fat: muscle, bone, organs, water. It's the number worth protecting while you diet, and the one your protein target should be set from.

Milo

Your lean mass will appear here.

How this is worked out

Lean mass is simply your weight minus your fat mass, so it depends entirely on knowing your body fat percentage:

lean mass = weight × (1 − body fat %)

If you don’t enter a body fat figure, one is estimated from your BMI, age and sex. That’s the weak link: BMI can’t distinguish a lean 90kg from a heavy 90kg, so a measured figure, or the tape method, will give you a much better answer.

The reason to track it: during a diet, total weight falling tells you very little on its own. Weight falling while lean mass holds steady is a diet working. Weight falling with lean mass going with it means the deficit is too steep, protein is too low, or you’ve stopped training.

The number is the easy part

Any calculator can hand you a figure. The hard bit is the next three months: eating roughly to it, noticing when it stops working, and changing it. Milo is a coach you text like a friend. You log what you ate in a sentence, and it learns your patterns, notices when your weight stalls, and adjusts the target from what you actually did rather than from a formula.

See how Milo works

Common questions

Why does protein get set from lean mass?

Because fat tissue doesn’t need feeding with protein. Setting a target from total bodyweight overshoots for anyone carrying more fat, sometimes by 40 or 50 grams a day, which is both expensive and unnecessary.

Can I gain lean mass while losing fat?

Yes, if you’re new to training, returning after a break, or carrying a good deal of fat. It’s much harder for a lean, experienced lifter, where the realistic goal in a deficit is holding what you have.

How fast can I actually build muscle?

Roughly 0.5-1% of bodyweight a month in the first year of proper training, and it slows sharply after that. Anyone promising faster is selling something. This is why patience beats aggression in a gaining phase.

Does lean mass include water?

Yes, and that’s why it swings day to day. Carbs and salt both pull water into muscle, so a high-carb day can add a kilo of apparent lean mass overnight. Look at the trend, not the reading.

Other calculators

These are estimates from population averages, not measurements of you. Individual metabolism varies by roughly ±10% around any formula. Nothing here is medical advice — if you have a health condition, are pregnant, or are under 18, talk to a healthcare professional before changing how you eat or train.