Three percentage formulas
Almost every percentage question is one of these three. The calculator above handles all of them.
| Question | Formula | Example |
|---|---|---|
| What is X% of Y? | Y × X ÷ 100 | 15% of 80 = 80 × 15 ÷ 100 = 12 |
| X is what percent of Y? | X ÷ Y × 100 | 18 is 72% of 25 |
| X is Y% of what number? | X ÷ (Y ÷ 100) | 12 is 15% of 80 |
How to calculate a percentage in your head
- 10%: move the decimal point one place left (10% of 80 is 8).
- 5%: half of 10% (4).
- 15%: 10% + 5% (8 + 4 = 12).
- 1%: move the decimal two places (0.8), then multiply for any whole percentage.
Percent of a percent
To find 20% of 50%, multiply the decimals: 0.20 × 0.50 = 0.10, which is 10%. This is how stacked discounts work, and why “20% off, then another 10% off” is 28% off in total, not 30%. The discount calculator does that for you.
Common mistakes
- Percentage points vs percent. Going from 10% to 15% is a rise of 5 percentage points, but a 50% increase. Use the percent change calculator for changes.
- Dividing the wrong way. “What percent of 25 is 18” is 18 ÷ 25, the part over the whole.
The three percentage questions
- What is 15% of 80? Multiply: 0.15 × 80 = 12.
- 12 is what percent of 80? Divide and multiply by 100: 12 ÷ 80 × 100 = 15%.
- 12 is 15% of what? Divide by the percentage: 12 ÷ 0.15 = 80.
Mental shortcuts
10% is the number with the decimal moved one place (10% of 240 = 24). 5% is half of that (12), 1% moves the decimal two places (2.4), and 15% is 10% + 5% (36). For tips, 20% is double 10%.
Related calculators
- Percent Change Calculator: Percentage increase, decrease and difference.
- Discount Calculator: Sale price, savings and stacked discounts.
- Margin & Markup Calculator: Profit margin, markup and selling price.
Questions people ask
How do I calculate a percentage of a total?
Divide the part by the total and multiply by 100. For 18 correct answers out of 25: 18 ÷ 25 × 100 = 72%.
How do I add a percentage to a number?
Multiply by 1 plus the percentage as a decimal: adding 8% tax to $50 is 50 × 1.08 = $54.
How do I work out a percentage on a phone calculator?
Type the part, divide by the whole, then multiply by 100. For 15% of 80, type 80 × 0.15.
Formula checked October 2026 · How we calculate