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Percent Body Weight Lost Calculator

Enter your starting and current weight to see your percent weight lost.

How it is calculated

The tool subtracts the current weight from the starting weight and divides by the starting weight: amount lost = start − current and percent lost = (start − current) ÷ start × 100. The percentage always uses the starting weight as the base, not the current one. Both weights must be in the same unit and above zero; you choose kilograms or pounds with the units setting, and anything other than kg is treated as pounds. The unit is only a label: the percentage does not depend on it. If the current weight is higher than the start, the result is negative, which means weight gained (80 kg to 84 kg shows -5.00%). The tool says nothing about whether the loss was healthy or intended, or over what period.

Worked example

Starting weight 92 kg, current weight 84.5 kg, units in kg. Amount lost: 92 − 84.5 = 7.50 kg. Percent lost: 7.5 ÷ 92 × 100 = 8.15%. The remaining weight shown is 84.50 kg. Note that 8.15% of the starting weight is not the same as 8.15% of the current weight, since the base is the 92 kg you began with.

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Common mistakes

  • Mixing units, for example a starting weight in pounds and a current weight in kilograms; the tool just subtracts the numbers.
  • Leaving the units on pounds while entering kilograms: the percentage is still right, but the labels say lb.
  • Confusing percent lost with percentage points of body fat; this tool is about total body weight only, not fat or muscle.
  • Expecting a positive number when weight went up; a negative percent means a gain, and the amount lost is also negative.
  • Using the current weight as the base. Losing 7.5 kg from 92 kg is 8.15%, not 8.9% (7.5 ÷ 84.5), which is the figure you would get by dividing by the wrong weight.

Sources

About this calculator

This calculator finds the percentage of body weight lost between a starting weight and a current weight, using the simple formula: (starting weight − current weight) / starting weight × 100. Expressing weight change as a percentage rather than a raw number of kilograms or pounds matters because losing the same 5 kg means something very different to a 100 kg person than to a 60 kg person.

This percentage has a specific clinical meaning: unintentional weight loss of more than about 5% within one month, or more than 10% within six months, is a recognized red flag used in malnutrition screening tools (such as MUST and MST) to identify patients who may need a nutritional assessment or workup for an underlying illness, since significant unplanned weight loss can signal anything from poor appetite to a more serious condition.

Outside the clinical setting, athletes in weight-class sports like wrestling, boxing, and rowing track percent weight lost when cutting weight before competition, and people on structured diets use it to measure progress against their starting point in a way that's comparable across different body sizes.

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