Percentage Calculator
The three percentage questions everyone asks — answered live.
The three formulas
X% of Y = Y × X ÷ 100 · A of B = A ÷ B × 100% · change = (B − A) ÷ A × 100%
Worked examples: 15% of 80 is 12. 30 out of 120 is 25%. Going from 50 to 65 is a 30% increase, while 65 back to 50 is only a 23.1% decrease — the base changed, which is the whole trick of percentages and the reason the third tab exists.
Frequently asked questions
How do I work out X percent of a number?
Multiply the number by the percentage and divide by 100: 15% of 80 = 80 × 15 ÷ 100 = 12. The first tab does this live — it is the version used for tips, marks, and quick discounts.
How do I work out what percent one number is of another?
Divide the part by the whole and multiply by 100: 30 out of 120 = 30 ÷ 120 × 100 = 25%. Teachers marking out of a total and anyone reading survey results use this direction constantly.
How is percentage increase calculated?
Change divided by the original, times 100: from 50 to 65 = (65 − 50) ÷ 50 × 100 = 30% increase. Decreases come out negative. The original (starting) value is always the denominator — that is the step people flip by mistake.
Why is a 50% increase not undone by a 50% decrease?
Because the base changes. 100 + 50% = 150, but 150 − 50% = 75, not 100. Percentages always act on their own current base — the asymmetry surprises everyone once and then never again.