How to Use the Percentage Calculator
This calculator handles the three percentage calculations people search for most often: finding a percentage of a number, finding what percent one number is of another, and finding the percentage increase or decrease between two values.
X% of Y
Use this when you know a percentage and a total, and need the resulting amount — for example, calculating a 20% discount on a $150 item, or figuring out a tip on a restaurant bill.
Worked example: 20% of $150 = (20 ÷ 100) × 150 = $30. If that's a discount, the final price is $150 − $30 = $120.
X is What % of Y
Use this when you have two numbers and want to know the relationship between them as a percentage — for example, "30 out of 120 correct answers is what percent?"
Worked example: 30 is what % of 120? (30 ÷ 120) × 100 = 25%.
Percentage Increase or Decrease
Use this to measure the percentage change between two values, such as a price that moved from $80 to $100, or a metric that dropped from last month to this month. The tool automatically tells you whether it's an increase or decrease.
Worked example: A price moves from $80 to $100: (100 − 80) ÷ 80 × 100 = 25% increase. If it instead moved from $100 down to $80: (80 − 100) ÷ 100 × 100 = -20%, i.e. a 20% decrease — note the percentage is not the same size in each direction, which is exactly why using the correct base value matters.
Why Percentages Trip People Up
Percentage change is often calculated incorrectly because people divide by the wrong number. The correct formula is (new − old) ÷ old × 100, using the original value as the base — not the new one. This calculator applies that formula automatically so you don't have to remember it.
Quick Reference Formulas
| You Want To Find | Formula |
|---|---|
| X% of Y | (X ÷ 100) × Y |
| X is what % of Y | (X ÷ Y) × 100 |
| % change from X to Y | ((Y − X) ÷ X) × 100 |
Frequently Asked Questions
How do I calculate a discount percentage?
Use the 'X% of Y' tab: enter the discount percentage and the original price to get the discount amount, then subtract it from the price.
What's the formula for percentage change?
(New Value − Old Value) / Old Value × 100. A positive result is an increase, a negative result is a decrease.
Can I calculate percentages with decimals?
Yes, the calculator accepts decimal numbers for precise results.
Is this calculator free to use?
Yes, completely free with unlimited calculations.
Why did I get a negative percentage?
A negative result in the change calculator means a decrease — for example, -20% means the value dropped by 20% from the original.