Percentage Calculator
The three percentage questions of everyday life — discounts, shares and changes — calculated live as you type, with the formula shown so you can follow along.
What is X% of Y?
Formula: Y × X ÷ 100
X is what percent of Y?
Formula: X ÷ Y × 100
Percentage increase / decrease
Formula: (new − old) ÷ old × 100
The three percentage cases explained
Nearly every percentage question in daily life is one of three patterns. Knowing which one you're looking at makes the math trivial:
X% of Y
"How much is the 20% discount on €150?" → 150 × 20 ÷ 100 = 30. Also covers tips, VAT and deposit amounts.
X is what % of Y
"I scored 45 of 60 points — what percentage is that?" → 45 ÷ 60 × 100 = 75%. Shares, quotas, test scores.
Change in %
"Rent went from €800 to €900 — by what percent?" → (900 − 800) ÷ 800 × 100 = +12.5%. Price and growth changes.
The classic percentage traps
The most common mistake is using the wrong base. A 50% increase followed by a 50% decrease does not return to the start: 100 → 150 → 75, because the second change is measured from the new, larger value. The same logic explains why a price that doubles has risen by 100% (not 200%), and why "percentage points" and "percent" are different things: an interest rate moving from 2% to 3% rises by one percentage point, but by 50 percent. The third calculator above handles the base correctly for you and shows the sign of the change.
Frequently asked questions
How do I calculate X percent of a number?
Multiply the number by the percentage and divide by 100. For example, 20% of 150 is 150 × 20 ÷ 100 = 30. The first calculator on this page does it live as you type.
How do I work out what percent one number is of another?
Divide the part by the whole and multiply by 100. For example, 45 out of 60 is 45 ÷ 60 × 100 = 75%.
How is percentage increase or decrease calculated?
Take the difference between new and old value, divide it by the old value and multiply by 100. Going from 80 to 100 is (100 − 80) ÷ 80 × 100 = +25%.
Why is a 50% increase not undone by a 50% decrease?
Because the base changes: 100 increased by 50% is 150, but decreasing 150 by 50% gives 75, not 100. Percentage changes always refer to their own starting value.