Lean Body Mass Calculator

Everything you are made of except fat — muscle, bone, organs and water. The number that decides how much you can eat and how strong you can get.

Sex

Units

Height (in)

Weight (lb)

Body fat % (optional)

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Lean body mass

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Fat mass

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Lean share

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Protein floor

Every formula, side by side

Formula	Lean mass	Notes

What your lean mass sets

Number	Value

Hold this number while you diet and you win. Set the protein that protects it → or see how it scores against your height →

If you enter a body fat percentage the result is calculated directly from it, which is the most accurate route. Without one, the Boer, James and Hume formulas estimate lean mass from height and weight alone and are shown for comparison; they assume an average build and will under-read on a heavily muscled lifter. General fitness information, not medical advice.

Why lean body mass is the number that matters

Lean body mass is everything you are made of except fat: muscle, bone, organs, connective tissue and water. It is the engine. It sets how many calories you burn at rest, how much protein you need, how strong you can get, and how well you hold up as you age. Scale weight is the sum of the engine and the cargo; lean mass is just the engine.

Two ways to get the number

If you know your body fat percentage, lean mass is simple subtraction and that is the most accurate route available without a lab. If you do not, the Boer, James and Hume formulas estimate it from height, weight and sex. They assume an average build, so they will under-read on anyone carrying serious muscle — which is most people who find this page. Get a tape measurement and use the direct method.

What it decides

Your protein floor is roughly 1.1 grams per pound of lean mass. Your resting metabolic rate, calculated the accurate way, comes straight off lean mass through the Katch-McArdle equation. And it sets what you will weigh at any given leanness — the table above shows your body weight at 12, 15 and 20 percent body fat, which is far more useful than a goal weight pulled out of the air.

The only thing a diet should cost you

When you lose weight, some of it is fat and some of it is lean mass. The entire job of a good diet is to tilt that split as far toward fat as possible. Enough protein, heavy lifting kept in place, and a deficit that is moderate rather than brutal. Run this calculator every four to six weeks: fat mass should fall and lean mass should hold. If lean mass is sliding, the diet is too aggressive, the protein is too low, or the training got soft.

Building it is slow, and that is normal

A trained lifter might add two to four pounds of genuine lean mass in a year. A beginner can do better in their first eighteen months. Anyone promising more than that is selling something. The upside is that the muscle you build is the most durable asset you own — it protects your metabolism, your joints and your independence decades later.