Three percentage questions, three quick formulas
"Percent" simply means "per hundred," so every percentage is a way of slicing a number into hundredths. Almost every real percentage problem is one of three shapes: finding a slice of a number, expressing one number as a slice of another, or measuring how much a number grew or shrank. The three tabs above map exactly onto those shapes — pick the one that matches your question and the answer updates as you type.
Worked example: 15% of 80, and 30 out of 120
Say a $80 jacket is marked down by 15%. To find the discount, multiply 80 by 15 and divide by 100: (80 × 15) ÷ 100 = 12, so you save $12 and pay $68. Now flip the question: if 30 students out of a class of 120 chose art, what share is that? Divide the part by the whole and multiply by 100: (30 ÷ 120) × 100 = 25%. The first case used the "X% of Y" tab; the second used "X is what % of Y" — same idea, different unknown.
The math behind each tab
How do I calculate a percentage of a number?
Multiply the number by the percentage and divide by 100. For example, 20% of 150 = (150 × 20) ÷ 100 = 30. This is the everyday case for discounts, tips, and tax.
How do I find what percentage one number is of another?
Divide the part by the whole and multiply by 100. For example, 30 is what percent of 150? (30 ÷ 150) × 100 = 20%. Use it for grades, test scores, and survey shares.
How do I calculate percentage change?
Subtract the old value from the new value, divide by the old value, then multiply by 100: ((New − Old) ÷ Old) × 100. A price moving from $80 to $100 is ((100 − 80) ÷ 80) × 100 = 25% increase; a positive result is growth and a negative result is a drop.
What is 15% of 200?
15% of 200 = (200 × 15) ÷ 100 = 30. Type 15 and 200 into the "X% of Y" tab above to confirm it instantly.
One trap to watch: points versus percent
The biggest mistake people make with percentages is confusing a percentage point with a percent change. If an interest rate rises from 4% to 5%, that is a 1 percentage-point increase but a 25% relative increase — very different numbers describing the same move, and the "% Change" tab reports the relative figure. Percent changes are also not reversible: a value that drops 20% and then rises 20% does not return to where it started. Finally, displayed results are rounded for readability, so for accounting or invoicing keep the full-precision figure rather than the rounded one shown here.