What this calculator tells you
A positive result means the new value is higher than the old value. A negative result means it is lower. The old value is the baseline, so changing which value is “old” changes the percentage.
Formula
Worked example
Useful for
- Comparing a price before and after a change
- Measuring growth or decline in traffic, usage, output or spending
- Checking how much a budget category changed between periods
Common mistakes
- Reversing the old and new values when the direction of change matters.
- Using zero as the old value. Percentage change from zero is undefined.
- Treating equal absolute changes as equal percentage changes when the starting values differ.
How to get a more useful estimate
Use inputs that match the decision you are actually making. If a price, rate, efficiency or usage figure can vary, run the calculator more than once with a low, expected and high case. Keep units consistent, avoid rounding intermediate values too early, and separate costs that this calculator does not include.
Frequently asked questions
Why does the old value matter?
The change is measured relative to the old value, which acts as the baseline.
Can percentage change be more than 100%?
Yes. If a value more than doubles, the percentage increase is greater than 100%.
What if the old value is zero?
A standard percentage-change calculation cannot divide by zero, so the result is undefined.
Related tools and guides
This calculator is a planning tool. For contracts, taxes, regulated charges, loans or other decisions with legal or financial consequences, verify the final figures and terms with the relevant provider or authority.