Guide
How to Calculate Percentages: 4 Formulas Everyone Needs
Most percentage questions use one of four formulas: X% of Y = Y × X ÷ 100; X as a percent of Y = X ÷ Y × 100; percentage change = (new − old) ÷ old × 100; and adding X% to Y = Y × (1 + X ÷ 100). For example, 20% of 150 is 30, and going from 80 to 100 is a 25% increase.
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The four formulas
| Question | Formula | Example |
|---|---|---|
| What is X% of Y? | Y × X ÷ 100 | 20% of 150 = 150 × 20 ÷ 100 = 30 |
| X is what % of Y? | X ÷ Y × 100 | 45 of 60 = 45 ÷ 60 × 100 = 75% |
| % change from A to B | (B − A) ÷ A × 100 | 80 → 100 = 20 ÷ 80 × 100 = +25% |
| Add X% to Y | Y × (1 + X ÷ 100) | $200 + 15% = 200 × 1.15 = $230 |
Skip the math
The Percentage Calculator does all four and shows the formula with your numbers.
Quick mental-math tricks
- 10%: move the decimal point one place left (10% of 85 = 8.5). Double it for 20%, halve it for 5%.
- 1%: move it two places (1% of 340 = 3.4). Multiply for any whole percentage.
- Swap trick: X% of Y = Y% of X. 8% of 50 is the same as 50% of 8 = 4.
- 15% tip: take 10%, then add half of it.
Common mistakes
- Reversing a percentage: a 20% rise followed by a 20% drop does *not* get you back to the start (100 → 120 → 96).
- Removing tax by subtracting: to remove 20% VAT from 120, divide by 1.2 (= 100), don’t subtract 20% (= 96). See the Sales Tax & VAT Calculator.
- Stacked discounts: 20% off then 10% off is 28% off in total, not 30%. Check with the Discount Calculator.
- Percent vs percentage points: a rate going from 4% to 5% is a 1 percentage-point rise but a 25% increase.
Frequently asked questions
How do I calculate a percentage of a total?
Divide the part by the total and multiply by 100. For example, 18 out of 24 is 18 ÷ 24 × 100 = 75%.
How do I work out a percentage decrease?
Subtract the new value from the old, divide by the old value and multiply by 100. 50 → 40 is a 20% decrease.
How do I find the original number before a percentage was added?
Divide by (1 + percentage ÷ 100). If $115 includes a 15% increase, the original was 115 ÷ 1.15 = $100.
Tools used in this guide
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