Calculators
How to Calculate Percentage
AllinOne Editorial · Published 2026-09-10 · Updated 2026-09-16
Percent means “per hundred”. Most day-to-day questions collapse into three formulas — and one division-by-zero trap.
A percentage is a number expressed as a fraction of 100. “15%” means 15/100. Once you remember that, the rest is multiplication or division.
1. What is X percent of Y?
X% of Y = (X ÷ 100) × Y. Example: 15% of 240 = 0.15 × 240 = 36. Sales tax, tips and exam scores all use this form.
2. X is what percent of Y?
(X ÷ Y) × 100. Example: 45 is 25% of 180 because 45 ÷ 180 = 0.25. Y cannot be zero.
3. Percentage increase or decrease
((new − original) ÷ |original|) × 100. From 80 to 100 is a 25% increase. If the new value is smaller, the result is negative and we call it a decrease.
The Percentage Calculator on this site runs all three formulas in the browser. Nothing is uploaded.
Common mix-ups
- A 50% decrease followed by a 50% increase does not return to the start (100 → 50 → 75).
- “20% off” is not the same as “pay 20%”. You pay 80%.
- Marks out of 40 converted to a percent use formula 2, not formula 1.
Frequently asked questions
Why can’t I calculate percent change from zero?+
You would divide by zero. A change from 0 to 10 is not a finite percentage; it is an undefined ratio.
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