How it works
A percentage is just a number out of 100, so the word “percent” literally means “per hundred”. This calculator covers the three questions people ask most often, each with its own mode you can switch between at the top.
What is X% of Y?
To find a percentage of a value, divide the percentage by 100 and multiply by the value:
- Result = Y × (X ÷ 100)
So 20% of 150 is 150 × 0.20 = 30. This is the formula behind discounts, tips, tax, and commission. If a $150 item is 20% off, the discount is $30 and you pay $120.
X is what percent of Y?
To express one number as a percentage of another, divide the part by the whole and multiply by 100:
- Result = (X ÷ Y) × 100
For example, 30 out of 150 is 30 ÷ 150 × 100 = 20%. This is how you turn a test score, a survey response, or a budget line into a percentage. If Y is zero the answer is undefined, so the calculator shows a short note instead of an error.
Percent change from X to Y
To measure how much a value has grown or shrunk, use the percent change formula:
- Result = (Y − X) ÷ X × 100
A move from 50 to 75 is (75 − 50) ÷ 50 × 100 = 50%, an increase. A move from 80 to 60 is (60 − 80) ÷ 80 × 100 = −25%, a decrease. The tool shows the size of the change and tells you whether it is an increase or a decrease. Because the old value sits on the bottom of the fraction, a starting value of zero has no defined percent change, and the calculator points that out rather than showing infinity.
A note on percentage points
Watch the difference between a percentage change and a change in percentage points. If an interest rate goes from 4% to 5%, that is a 1 percentage point rise, but a 25% relative increase. Both statements are correct, they just measure different things, so it pays to be clear about which one you mean.
Rounding
Results are rounded to a sensible number of decimal places, up to four, with any trailing zeros trimmed so the answer stays clean and easy to read.