How it works
One percentage tool with five modes: X% of Y, X as a percentage of Y, percentage change, increase/decrease by X%, and reverse percentage.
Pick the mode that matches your question; only the fields that mode needs are shown. Results update when you submit the form, and each mode also explains the value it shows — for example the difference between two values in change mode, or the original value in reverse mode. Division-by-zero cases (like a zero whole or a −100 % reverse) are reported instead of returning Infinity.
Formula
Of: (X ÷ 100) × Y · Is-of: (X ÷ Y) × 100 · Change: ((B − A) ÷ A) × 100 · Adjust: V × (1 + X ÷ 100) · Reverse: F ÷ (1 + X ÷ 100).
X- Percentage entered by the user (%)
Y- Value the percentage applies to (number)
A, B- Starting and ending values for a change (number)
V, F- Value to adjust / final value after a change (number)
Examples
- What is 25% of 200?Mode: X% of Y · percent 25 · value 20050
- After a 20% increase a value is 120Mode: reverse percentage · percent 20 · final value 120Original value 100
Frequently asked questions
What is the difference between percentage change and percentage points?
A change from 10 % to 15 % is a 5 percentage-point rise but a 50 % relative increase. This tool reports relative percentage change; use percentage points when comparing rates directly.
Can I use negative percentages?
Yes. In increase/decrease mode a negative percentage reduces the value, and negative values are handled correctly in the other modes too.
How does reverse percentage work?
It divides the final value by 1 + the percentage, recovering the original amount before the change — for example 120 after a 20 % increase goes back to 100.
Why does it reject a zero whole value?
Dividing by zero has no finite percentage. The tool shows a clear message instead of returning Infinity or NaN.
Does 100% change have a special case?
A 100 % increase doubles a value; a −100 % change would reduce it to zero, so the reverse calculation has no finite original value and is rejected.
How to read the result
Read the mode label with the result: a percentage of something, a share, a change, or a recovered original value. In change mode the difference is also shown, which makes the size of the move concrete.
Limitations
Supports the five modes listed here; it is not a full spreadsheet. Very large values are bounded, and rounding is applied to the displayed result.
Source: Elementary percentage identities; exact arithmetic with guarded division.