Start with the percentage of starting weight
Subtract starting weight from ending weight, divide by starting weight, and multiply by 100. For the weekly rate, divide that percentage by the number of days and multiply by seven.
Read how to calculate weight change percentage for the signed formula, examples, interpretation, and research limits.
Example: 0.70 lb lower in seven days
A starting average of 180.14 lb and ending average of 179.44 lb differ by -0.70 lb. Dividing -0.70 by 180.14 and multiplying by 100 gives -0.39%. Because the averages are seven days apart, the average weekly result is also -0.70 lb and -0.39% per week.
Average first, then compare
One scale reading can move with water, digestion, sodium, carbohydrate intake, training, illness, menstrual-cycle changes, and timing. Use the weekly average weight calculator to produce more comparable endpoints.
Why a percentage travels better than a number
The same absolute change means different things at different body sizes. A kilogram a week is a much larger proportional change for a 55kg person than for a 110kg one, which is why rate-of-change guidance is usually expressed as a percentage of body weight rather than a fixed amount.
Expressing it that way also makes your own progress comparable across time. A rate that was moderate at a heavier weight can be aggressive at a lighter one, even though the number on the scale moved the same amount.
Reading the result honestly
A percentage calculated from two single weigh-ins inherits all the noise in both. Use multi-day averages at each end, taken under comparable conditions, or the figure mostly measures water and timing. The weekly average weight calculator produces those endpoints.
A percentage also says nothing about what changed. It describes scale weight, and scale weight includes water, glycogen and gut contents alongside tissue. Look at direction across several comparable periods rather than reacting to one figure, and be slower to act on a surprising result than a predictable one.
The sign describes direction
A negative percentage means the ending average is lower. A positive percentage means it is higher. Neither result identifies fat, muscle, water, or another component of the change.
This tool does not set a safe rate. Children, pregnancy, eating-disorder treatment, unexplained change, medication-related change, and medical fluid management need qualified clinical guidance.