How to use the percentage calculator
Pick the calculation you need — “X% of Y”, “X is what percent of Y”, or “percent change from X to Y” — then type your two numbers. The result updates instantly, so you can try a few values before you settle on one.
Three ways to calculate a percentage
X% of Y answers questions like “what is 20% of 150?” The formula is (X / 100) × Y. For 20% of 150, that’s 0.20 × 150 = 30. This is the mode you want for tips, taxes, and discounts.
X is what percent of Y flips the question around: “12 is what percent of 40?” Here the formula is (X / Y) × 100, so (12 / 40) × 100 = 30%. Use this when you already know two amounts and need the ratio between them, like test scores out of a total or a portion of a budget.
Percent change from X to Y tells you how much a value grew or shrank: ((Y − X) / X) × 100. Going from 80 to 100 is ((100 − 80) / 80) × 100 = 25% — a 25% increase. A drop from 100 to 80 is a 20% decrease, not 25%, because the base value is different. This asymmetry trips people up constantly, so it’s worth double-checking which number is your starting point.
Real-life examples
- Discounts: A $60 jacket at 25% off costs
60 − (0.25 × 60) = $45. - Test scores: Getting 42 out of 50 questions right is
(42 / 50) × 100 = 84%. - Raises: A salary going from $50,000 to $53,500 is a
((53,500 − 50,000) / 50,000) × 100 = 7%raise. - Tips: 18% tip on a $34 bill is
0.18 × 34 = $6.12.
A common mistake: percent vs. percentage point
If an interest rate moves from 5% to 7%, that’s a 2 percentage point increase, but a ((7 − 5) / 5) × 100 = 40% relative increase. News headlines often blur this distinction, and it changes the meaning of the number a lot — 2 points sounds small, 40% sounds huge, and both are technically correct depending on what’s being measured. When you read or report a percentage change, be clear about which one you mean.
Another frequent slip is applying a percentage increase and then the same percentage decrease and expecting to land back where you started. A 20% increase followed by a 20% decrease doesn’t cancel out, because the second calculation uses a larger starting number. $100 up 20% is $120; $120 down 20% is $96, not $100.