A percentage is a number expressed as a fraction of 100, used for increases, decreases, discounts and shares of a total. What is 15% of 80? 12 is what percent of 80? Both directions, plus percent change - with the arithmetic shown so you can check it yourself.
Almost every percentage problem is one of three shapes. "What is 15% of 80?" multiplies: 80 × 0.15 = 12. "12 is what percent of 80?" divides: 12 ÷ 80 = 0.15 = 15%. "What's the change from 80 to 92?" compares: (92 − 80) ÷ 80 = 15% increase. The toggle above switches between them, and the answer always shows the arithmetic so you can check it - or reuse it without the tool next time.
Percent change is measured against the starting value, which is why going from 80 to 100 is a 25% increase, but going back from 100 to 80 is only a 20% decrease. Same 20-point gap, different percentages - because the base you divide by changed. This tool always uses the "from" value as the base and says so in the result.
Working out a tip or splitting a bill? The unit price comparator handles which-deal-is-cheaper. Comparing grades? The grade calculator works in weighted percentages.
Multiply the number by the percentage and divide by 100. Fifteen percent of 240 is 240 × 15 ÷ 100 = 36.
Subtract the original value from the new one, divide by the original, and multiply by 100. Going from 40 to 50 is (50 - 40) ÷ 40 × 100 = 25% increase.
A rise from 5% to 7% is a 2 percentage point rise, but a 40% increase. Mixing the two is a common source of misleading statistics.