Body composition: fat, lean mass and water
Written by The Kalcule Team · Last updated 2026年8月18日
Scale weight is the sum of fat, lean tissue and water. Composition tools try to split that sum. Understanding the split explains why two people at the same BMI look different.
The three buckets
Fat mass, fat-free mass and water (which sits mostly inside fat-free mass) all move. A salty meal is not fat gain. A hard training week can add glycogen and water.
Why BMI and composition diverge
High lean mass raises BMI without raising fat percentage. Low lean mass can hide high fat at a “normal” BMI. Use BMI, waist, and a composition estimate together.
Tools on this site
BMI, Navy body-fat and Boer lean-mass calculators are complementary estimates. None of them is a DEXA. Read each page’s limitations.
Tracking without obsession
Monthly composition plus weekly waist and training logs beat daily scale drama. If tracking is taking over your life, step back.
常见问题解答
Body composition basics.
Yes. Glycogen binds water; sodium and hormones shift fluid. It is not fat.
Rare over short periods if you lose a lot of water or muscle. Usually weight and fat move together over months.
It depends on age, sex and sport. Population ranges exist; your clinician or coach can interpret yours.
Not with adult tools. Paediatric assessment is different.
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