What is X% of Y?
To find X percent of Y, turn the percent into a decimal and multiply: result = Y × X ÷ 100. "Percent" means "per hundred", so 15% is 15 out of every 100, or 0.15.
Worked examples
- 15% of 80: 80 × 0.15 = 12.
- 7.5% of 240: 240 × 0.075 = 18.
- 120% of 50: 50 × 1.2 = 60. Percentages above 100 simply give a result bigger than the starting number.
- 0.5% of 3,000: 3,000 × 0.005 = 15. Watch the decimal: 0.5% is 0.005, not 0.05.
Shortcuts you can do in your head
- 10%: move the decimal point one place left. 10% of 86 is 8.6.
- 1%: move it two places. 1% of 86 is 0.86. Build other rates from these: 3% is three times 1%.
- 5%: half of 10%. 5% of 86 is 4.3.
- 25%, 50%, 75%: a quarter, a half, three quarters.
- Swap the numbers: X% of Y always equals Y% of X. 8% of 50 is hard; 50% of 8 is 4, and that is the same answer.
The reverse question: "Y is what percent of X?"
Divide the part by the whole and multiply by 100: percent = part ÷ whole × 100. If you scored 42 out of 60, that is 42 ÷ 60 × 100 = 70%. PercentSnap has a separate mode for this, so you do not have to rearrange the formula.
Finding the whole when you know the part
Sometimes you know the result and the rate but not the starting number: "18 is 15% of what?" Divide the part by the decimal rate: 18 ÷ 0.15 = 120. A quick check: 15% of 120 is 18.
Everyday uses
- Discounts: a 30%-off sticker on a 65 item takes off 65 × 0.30 = 19.50, so you pay 45.50. Faster: pay 70% of the price, 65 × 0.70 = 45.50.
- Sales tax: 8.25% tax on 40 is 40 × 0.0825 = 3.30, total 43.30. Or multiply by 1.0825 in one step.
- Budget shares: if rent is 1,400 of a 4,200 monthly income, rent is 1,400 ÷ 4,200 × 100 ≈ 33.3% of income.
- Grades and scores: the "what percent" mode above.
Mistakes to avoid
- Stacking discounts by adding them. 20% off and then an extra 10% off is not 30% off. You pay 0.8 × 0.9 = 0.72 of the price, which is 28% off.
- Confusing percent with percentage points. A rate rising from 4% to 5% is a 1 percentage-point rise but a 25% increase in the rate itself.
- Misplacing the decimal for small rates such as 0.25% (0.0025).
If you would rather not do it by hand, the calculator's "X% of Y" mode gives the answer as you type and keeps your last calculations in your browser.