How it works
The first formula multiplies by X/100. The second divides the part by the whole and multiplies by 100. The third takes the difference, divides by the absolute original value and multiplies by 100, so the sign shows direction.
- Results are rounded to four decimals; the formula is shown next to each answer.
Why it matters
Discounts, growth, tax and performance figures all use percentages, and they are routinely compared as if the base never changes. Seeing the three separate formulas makes the correct base explicit before the number reaches a slide.
A worked example
Twenty percent of 150 is 30. Thirty is 20% of 150. A move from 100 to 125 is (125−100)/100 = +25%, while 125 to 100 is −20% because the base changed.
Common mistakes
- Confusing percentage points with percent change.
- Dividing by the new value instead of the original.
- Ignoring that a negative base flips the sign.
Practical tips
When someone says "up 5%", ask whether they mean percentage points or relative change; the difference matters most for rates that are already small. Chain changes by multiplying factors, not by adding percentages.
How to use it
- Open the tool. Everything runs in your browser tab.
- Paste or type your input, then choose the action you need.
- Check the result and copy it with one click.
Common questions
Which formulas does it use?
X/100 × Y, X/Y × 100, and (Y − X)/|X| × 100 for change. Each formula is shown next to the result.
Why is my change negative?
A decrease produces a negative percentage. The sign shows direction, not an error.
How are results rounded?
Displayed to four decimal places; the calculation itself is not rounded between steps.
Is this tool really free?
Yes. There is no account, no paywall and no usage limit. Tools App is funded by aumenso, the studio behind it.
Is anything I enter uploaded?
No. The tool runs in your browser tab; what you enter is never sent to us and never stored.