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Body Recomposition Change Calculator

The scale hides recomposition. Enter before and after weight plus body fat to see fat and muscle separately.

How it is calculated

For each check the tool splits weight into two parts: fat = weight × BF% ÷ 100 and lean = weight − fat. It then subtracts the start values from the end values, so a plus sign means a gain and a minus sign a loss, in kilograms. It reports the change in fat, in lean mass and in total weight. The summary label uses a 0.2 kg threshold: fat down and lean up is recomposition; fat down with lean unchanged is a clean cut; fat down with lean also down is a cut; both up is a bulk; only fat up is fat gain; anything else is stable. Lean mass here means everything that is not fat, including water and organs, not muscle alone. Both weights must be above zero and both percentages between 0 and 100.

Worked example

At the start you weigh 82 kg at 22% body fat, and later 81 kg at 18%. Start: fat = 82 × 0.22 = 18.04 kg, lean = 63.96 kg. End: fat = 81 × 0.18 = 14.58 kg, lean = 66.42 kg. Fat change = 14.58 − 18.04 = -3.46 kg; lean change = 66.42 − 63.96 = +2.46 kg; weight change = -1.00 kg. Fat is down by more than 0.2 kg and lean is up by more than 0.2 kg, so the summary reads recomposition. The scale shows only 1 kg, which hides the 3.46 kg of fat lost.

Try this example →

Common mistakes

  • Measuring body fat with two different methods at the start and the end (for example calipers first, an impedance scale later); the method difference can look like a real change.
  • Taking the second impedance reading at a different time of day or hydration level, so water shifts show up as lean mass gain or loss.
  • Calling the lean change muscle: it also contains water, glycogen, bone and organs.
  • Entering the percentage as a fraction such as 0.22 instead of 22.
  • Over-reading a small change: differences under 0.2 kg are reported as stable, and a one-point difference in body-fat percentage is often within measurement error.

About this calculator

Body recomposition — losing fat and gaining muscle at the same time — is easy to miss on a scale because the two changes can roughly cancel out in total weight, even though body composition shifted meaningfully. Tracking it requires two body-composition checks (weight plus body fat percentage, from calipers, a scale with bioelectrical impedance, or a DEXA scan) at two points in time: fat mass = weight × body fat %, and lean mass = weight − fat mass, calculated at both checkpoints and then compared.

Strength coaches and people on a recomposition-focused program (common among beginners and people returning to training after time off, since both fat loss and muscle gain can happen simultaneously at that stage) use this comparison because scale weight alone is a poor progress metric during recomp — someone who lost 4 lbs of fat and gained 3 lbs of muscle only sees a 1 lb change on the scale despite real, meaningful progress.

Compare your before and after weight and body fat percentage to see exactly how many kilograms of fat went and how much lean mass came on, even when the scale barely moved.

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