Percentage formulas
"Percent" means "per hundred", so 15% is 15/100 = 0.15. Each calculator above uses one of these formulas:
Examples: 15% of 80 is 12. 30 is 25% of 120. Going from 50 to 65 is a 30% increase.
Percentage change vs. percentage points
If an interest rate rises from 4% to 5%, that's a rise of 1 percentage point but a 25% percentage change. Use percentage points when comparing two percentages directly.
Increases and decreases aren't symmetric either. A 50% drop followed by a 50% gain leaves you at 75% of where you started, not back at 100%.
Percent, decimal and fraction
| Percent | Decimal | Fraction |
|---|---|---|
| 1% | 0.01 | 1/100 |
| 5% | 0.05 | 1/20 |
| 10% | 0.1 | 1/10 |
| 12.5% | 0.125 | 1/8 |
| 20% | 0.2 | 1/5 |
| 25% | 0.25 | 1/4 |
| 33⅓% | 0.333… | 1/3 |
| 40% | 0.4 | 2/5 |
| 50% | 0.5 | 1/2 |
| 66⅔% | 0.666… | 2/3 |
| 75% | 0.75 | 3/4 |
| 100% | 1 | 1/1 |
To turn a percent into a decimal, divide by 100. To turn a decimal into a percent, multiply by 100.
Percentages in your head
- 10%: move the decimal point one place left. 10% of 64 is 6.4.
- 5%: half of 10%. 5% of 64 is 3.2.
- 15%: 10% plus 5%. 15% of 64 is 6.4 + 3.2 = 9.6.
- 20%: double 10%. 20% of 64 is 12.8.
- 1%: move the decimal two places left. 1% of 64 is 0.64.
- Swap them: X% of Y always equals Y% of X. 8% of 50 is the same as 50% of 8, which is 4.
Frequently asked questions
How do I add a percentage to a number?
Multiply by (1 + percent ÷ 100). For example, 200 plus 8% is 200 × 1.08 = 216. To subtract a percentage, multiply by (1 − percent ÷ 100).
How do I reverse a percentage increase?
Divide by (1 + percent ÷ 100). If a price is $216 after an 8% increase, the original was $216 ÷ 1.08 = $200.
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