About the Percentage Calculator
The Percentage Calculator handles the three most common percentage questions in one place: what is X percent of Y, X is what percent of Y, and by what percent did a value change between an old and new number.
Results update as you type. The formulas are simple: part = percentage ÷ 100 × number; share = X ÷ Y × 100; change = (new − old) ÷ |old| × 100.
How to use it
- For percent of a number, enter the percentage and the number.
- For what percent, enter X and Y.
- For change, enter the starting and new value; the tool shows the percent increase or decrease.
Worked examples
20% of 150 = 30. A mark of 42 out of 60 is 70%.
A price rising from 80 to 100 is a 25% increase, but falling from 100 back to 80 is a 20% decrease, because the base differs.
If the old value is zero, percentage change is not defined.
Real-world examples
Marks. 42 out of 60 is 70 percent.
Salary rise. From 3,000 to 3,300 is a 10 percent increase.
Sales tax or VAT. 15 percent of 80 is 12.
Business metrics. Monthly visitors rising from 8,000 to 10,000 is up 25 percent.
Common mistakes
Mixing up 'percent of' and 'percent change'. Reversing the old and new values, which flips an increase into a decrease. Adding a percentage rise and a percentage fall of the same size and expecting to return to the start. Treating percentage points and percent as the same thing.
How it works
Three formulas run side by side: part = percentage ÷ 100 × number; share = X ÷ Y × 100; change = (new − old) ÷ |old| × 100. Dividing by zero is detected and shown as 'Not defined' instead of a misleading number.
Frequently asked questions
Why is increase different from decrease?
Each is measured against a different starting value.
Can I use decimals?
Yes.
What if Y is zero?
The tool shows Not defined.