How this calculator works
The starting weight is the denominator. Both measurements are converted to kilograms, so pounds and kilograms give the same percentage for the same physical weights. A negative result represents gain. This arithmetic does not distinguish fat, muscle or fluid, and does not decide whether a change is intentional or healthy. The result is an estimate for information only and is not medical advice.
Formula
Percentage lost = 100 x (starting - current)/starting. Current = starting x (1 - loss fraction). Starting = current/(1 - loss fraction). Mass lost = starting - current.
Worked example
A change from 100 kg to 90 kg is a 10% loss. From 80 kg to 84 kg, the result is -5%, meaning a 5% gain.
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Frequently asked questions
Is the denominator my current weight?
No. Use the starting weight when expressing change from that starting point.
Why is my result negative?
Your current weight exceeds the starting weight, so the signed loss is negative.
Does a given percentage mean I should lose more?
No. The calculator does not set a goal. Unintentional changes may warrant discussion with a health professional.