Percentage calculator
Find a percentage of a number, and increase or decrease a number by a percentage.
Formula
How it works
A percentage is a fraction of 100. Multiply the number by the percentage and divide by 100 to get that share. Add it to the number to increase, subtract it to decrease.
Worked example
The same calculation with the default values:
- Percentage 15 %
- Number 200
- 200 × 15 ÷ 100 = 30
- 200 + 30 = 230
- 200 − 30 = 170
Result: Percentage of the number: 30 · Number increased by the percentage: 230 · Number decreased by the percentage: 170
Common mistakes
- Confusing "percent of" with "percent change" - a price rising from 100 to 120 is a 20% increase, but 20% of 120 is 24, not 20; these use different formulas and different base values.
- Forgetting that percentage change is always relative to the original (starting) value, not the new value - a 50% increase followed by a 50% decrease does not return to the starting number.
- Dropping the ×100 step when converting a decimal ratio into a percentage, or adding an extra one - this is the single most common source of answers that are off by a factor of 100.
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