How this calculator works
The increase is the starting value multiplied by the percentage as a fraction. Add that amount to the starting value to get the new value. The reverse calculation compares the difference with the original amount. A negative rate is accepted and applies a decrease.
Formula
Increase = old × r/100. New = old + increase. Rate (%) = (new - old) / old × 100.
Worked example
A 15% increase on 200 adds 30, giving 230. Starting with 200 and ending with 230 also gives an increase rate of 15%.
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Frequently asked questions
Does a negative percentage work?
Yes. Entering -25% on 200 adds -50, giving 150.
Are two 10% increases a 20% increase?
No. Each increase acts on the current amount. Two successive 10% increases multiply the start by 1.21, for a total increase of 21%.
Why can an increase from a negative starting value show a negative rate?
The percentage is measured against the signed starting value. From -100 to -50 the gap is 50 and the start is -100, so the result is -50%.