Percentage Calculator
Four common percentage questions in one tool: percent of a value, ratio as a percentage, percentage change, and percentage increase or decrease.
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What does percent actually mean?
"Per cent" literally means "per hundred". A percentage is a fraction with an implicit denominator of 100. So 25% is 25/100 = 0.25, and "what is 25% of 200" is the same question as "what is 0.25 × 200".
The four canonical questions
- x% of a value. Multiply the value by x ÷ 100. So 15% of 80 is 80 × 0.15 = 12.
- What % is a of b? Divide a by b and multiply by 100. So 30 of 200 is 30/200 × 100 = 15%.
- Percentage change. Subtract old from new, divide by old, multiply by 100. So 80 to 100 is (100 − 80)/80 × 100 = 25% increase.
- Add or subtract a percentage. Multiply the value by (1 ± percent/100). So 200 + 15% is 200 × 1.15 = 230, and 200 − 15% is 200 × 0.85 = 170.
Common pitfalls
Two errors come up over and over. First: a 50% increase followed by a 50% decrease does not bring you back to the start. 100 becomes 150, then 150 × 0.5 = 75 — a 25% net loss. Second: percentages do not add. A 10% bonus on top of a 10% raise is not 20% — it is (1.1)(1.1) − 1 = 21%. The calculator handles both correctly.