Lean body massLBM · Fat-free mass

Everything in your body that isn't fat, chiefly muscle, bone and water.

Lean body mass is your total weight minus your fat mass: muscle, bone, organs, connective tissue and the water in all of it. It is the counterpart to body fat percentage, and the two are simply different views of the same measurement.

It matters because it is the part of you that does things. Muscle produces force, stores glucose and protects function in later life, and lean mass is the strongest single predictor of resting energy expenditure, which is why two people of the same weight can have noticeably different maintenance calories.

It is also the better denominator for a lot of arithmetic. Protein targets, energy needs and several clinical calculations track lean mass more closely than total weight, because fat tissue is far less metabolically demanding per kilogram.

Do not read it as pure muscle. Water alone accounts for a large share, and it swings with carbohydrate intake, sodium, hydration, the menstrual cycle and recent training. A three kilogram jump in a week is a fluid story, not a muscle one, and a scale that reports lean mass daily is mostly reporting your hydration.

Measurement is approximate whatever the device claims. A DEXA scan is the practical reference and still varies between machines and between visits; bioimpedance scales are heavily affected by hydration; circumference equations such as the Boer or Navy formulas are cheap and rough. Pick one method, use it consistently, and read the trend rather than any single figure.

During a diet, holding lean mass is the whole game. Adequate protein and continued resistance training are what decide whether the weight you lose is fat or the tissue you spent years building, which is also why the same kilogram lost can leave two people looking entirely different.

In practice, use our lean body mass calculator for the estimate and the FFMI calculator to see how that mass compares for your height.

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