Percentage Calculator
Answer the three most common percentage questions instantly, then jump to a dedicated calculator for increases, decreases, differences, tips or discounts.
What is X% of a number?
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X is what percent of Y?
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X is Y% of what number?
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How to calculate
Percentages describe a part relative to 100, so all percentage math traces back to two moves: multiplying a value by a percentage (percent × value ÷ 100), or dividing one value by another and multiplying by 100. This page covers the three most common questions: finding X% of a number, finding what percent one number is of another, and reversing a percentage to find the original whole.
To find X% of a number by hand, convert the percent to a decimal by dividing by 100, then multiply by the value — 15% of 40 becomes 0.15 × 40 = 6. To find what percent one number is of another, divide the part by the whole and multiply by 100 — 6 out of 40 is (6 ÷ 40) × 100 = 15%. Reversing that, if 6 is 15% of some number, divide 6 by 0.15 to get 40 back.
The most common mistake is dividing by the wrong number — the 'whole' always goes in the denominator, whether that's the original price, the total group size, or the full amount. Also worth knowing: a percentage is not the same as a percentage point, so 'up 5 percentage points' and 'up 5%' can mean very different things when the starting percentage is far from zero.
Examples
- 15% of 80 = (15 ÷ 100) × 80 = 12.
- 12 is what percent of 80? (12 ÷ 80) × 100 = 15%.
- 18 is 30% of what number? 18 ÷ (30 ÷ 100) = 60.
- 45 is what percent of 180? (45 ÷ 180) × 100 = 25%.
Frequently asked questions
- How do I calculate a percentage of a number?
- Divide the percentage by 100 and multiply by the number. For example, 20% of 50 is (20 ÷ 100) × 50 = 10.
- How do I find what percentage one number is of another?
- Divide the first number by the second and multiply by 100. For example, 30 out of 120 is (30 ÷ 120) × 100 = 25%.
- How do I find a number when I know a percentage of it?
- Divide the known part by the percentage expressed as a decimal. For example, if 18 is 30% of a number, divide 18 by 0.3 to get 60. This 'reverse percentage' is the same formula the third calculator mode uses, just isolating a different variable.
- What's the difference between a percent and a percentage point?
- A percent is a relative measure (a change from one value), while a percentage point is an absolute difference between two percentages. Going from 20% to 25% is a 5-percentage-point rise, but a 25% relative increase ((25−20)/20×100). Mixing the two is a common reporting error.