Percentage Calculator
Almost every everyday percentage question is one of three: “what is 15% of 200?”, “45 is what percent of 180?”, or “30 is 15% of what?”. This percentage calculator handles all three — pick the mode, enter two numbers, get the answer with the formula shown.
Percentages are just fractions with a denominator of 100, which is why every one of these reduces to one multiplication or division.
Enter valid values to see the results.
Formula
The word “of” almost always means multiply, and “percent” means “per hundred”. Convert the percentage to a decimal (15% = 0.15) and the rest is ordinary multiplication or division.
What you need
- Mode 1 — X% of Y: tips, discounts, tax amounts. X is the percentage, Y is the base amount.
- Mode 2 — X is what % of Y: test scores, completion rates, market share. X is the part, Y is the whole.
- Mode 3 — X is Y% of what: reverse problems, e.g. “$30 is 15% of the bill — what was the bill?”
Worked example
Practical tips
- To find 10% of anything, move the decimal point one place left: 10% of 240 = 24. Build other percentages from there.
- 50% is a half, 25% a quarter, 20% a fifth, 1% a hundredth — mental shortcuts beat calculators for round numbers.
- Percentages are reversible: 8% of 25 equals 25% of 8 (= 2). Use whichever is easier mentally.
- For “percent of a percent”, multiply: 10% of 30% is 3% of the original.
Quick percentage reference
| Percentage | Fraction | Decimal | Shortcut |
|---|---|---|---|
| 10% | 1/10 | 0.1 | ÷ 10 |
| 20% | 1/5 | 0.2 | ÷ 5 |
| 25% | 1/4 | 0.25 | ÷ 4 |
| 50% | 1/2 | 0.5 | ÷ 2 |
Common mistakes
- 1 Dividing by the wrong number in “what percent” problems — always divide the part by the whole.
- 2 Forgetting to convert % to a decimal: 15 × 200 = 3,000 is wrong; 0.15 × 200 = 30 is right.
- 3 Adding percentages of different bases: 10% of your salary plus 10% of your rent is not 10% of anything meaningful.
Frequently asked questions
How do I calculate a percentage of a number?
Convert the percentage to a decimal and multiply: 35% of 80 = 0.35 × 80 = 28.
How do I find what percent one number is of another?
Divide the part by the whole and multiply by 100: 17 out of 68 = 17 ÷ 68 × 100 = 25%.
How do I find 100% if I know a percentage part?
Divide the part by the percentage as a decimal: if 45 is 30% of the total, the total is 45 ÷ 0.30 = 150.
Can a percentage be over 100%?
Yes — it just means more than the whole: 150% of 80 is 120. Common in growth, markups and comparisons.