Percentage Calculator
Four percentage questions in one place: what is X% of Y, X is what percent of Y, X is Y% of what number, and the percentage change between two values.
How it works
A percentage is simply a fraction out of 100. “18%” means 18 per 100, or 0.18 as a decimal — so every percentage question is a small multiplication or division once you know which number plays which role.
Pick the question that matches what you actually have. Most mistakes come from using the percentage-change formula when you meant a plain percentage of a number, so the calculator keeps the four cases separate and tells you which one you are answering.
The formula
X% of Y
result = (X ÷ 100) × Y
18% of 750 → (18 ÷ 100) × 750 = 135
X is what % of Y
result = (X ÷ Y) × 100
135 out of 750 → (135 ÷ 750) × 100 = 18%
X is Y% of what
result = (X × 100) ÷ Y
135 is 18% of → (135 × 100) ÷ 18 = 750
Percentage change
result = ((new − old) ÷ |old|) × 100
The sign matters: a negative result is a decrease.
Worked examples
| Scenario | Working | Result |
|---|---|---|
| A 750 bill with 18% service added | (18 ÷ 100) × 750 | 135 |
| Scored 42 out of 60 on a test | (42 ÷ 60) × 100 | 70% |
| Paid 90 after a 25% discount | (90 × 100) ÷ 75 | 120 before discount |
| Rent went from 1,200 to 1,320 | ((1320 − 1200) ÷ 1200) × 100 | +10% |
When you'd use it
- Adding service charge, tax or commission to a price
- Turning a mark out of any total into a percentage
- Finding the original price when you only know the sale price
- Comparing this month's figure against last month's
Common questions
How do I calculate 18% of 750?
Divide 18 by 100 to get 0.18, then multiply by 750. That gives 135. In the calculator, use the “X% of Y” mode with 18 and 750.
What is the difference between percentage change and percentage difference?
Percentage change has a clear starting point and divides by that original value. Percentage difference compares two values with no natural “before”, so it divides by the average of the two. They give different answers for the same pair of numbers.
Why can't I calculate a percentage change from zero?
The formula divides by the original value, and dividing by zero is undefined. If something went from 0 to 50, the honest description is “new”, not a percentage increase.
How do I reverse a percentage to find the original price?
If an item cost 90 after 25% off, you paid 75% of the original. Divide 90 by 0.75 to get 120. The “X is Y% of what number” mode does this for you — enter 90 and 75.