Percentage Calculator
Three modes for all percentage calculations.
What is X% of Y?
Percentage calculations: the three cases you encounter every day
Almost every percentage question in real life falls into one of three patterns. “What is 20% off $85?” is X% of Y. “My score was 42 out of 55: what is that as a percentage?” is X is what percent of Y. “Sales went from 1,200 to 1,500: how much did they grow?” is percentage change. Recognising which type you are dealing with is the first step; this calculator handles the arithmetic.
The formula display in the results is intentional: it shows you exactly how the answer was computed so you can verify it, reproduce it in a spreadsheet, or explain it to someone else. Many percentage mistakes happen because people apply the wrong formula: for example using a simple difference instead of dividing by the base for percentage change. Seeing the formula helps build intuition for which approach applies.
How to use the percentage calculator: step by step
- 1Choose your calculation mode
Select one of three tabs: 'X% of Y' to find a percentage of a number, 'X is what % of Y' to find the percentage relationship between two numbers, or '% Change' to find the percentage increase or decrease between values. - 2Enter your values
Type the numbers into the input fields. The calculator accepts decimals and negative numbers. Results update automatically as you type: no button to press. - 3Read the result
The large number at the bottom of the panel is your answer. Below it is the formula used, expressed in plain math notation so you can verify the calculation. - 4Check the formula
The formula line shows exactly how the result was computed: for example 15 × 200 ÷ 100 = 30. This is useful for understanding the math or reproducing it in a spreadsheet. - 5Review recent calculations
The Recent Calculations panel below the input keeps the last five results with their expressions. Use it to compare values without re-entering them.
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Frequently Asked Questions
- How do I calculate X% of Y?
- Multiply Y by X and divide by 100. For example, 15% of 200 = (200 × 15) / 100 = 30. Use the 'X% of Y' tab and enter X as the percent and Y as the number.
- How do I find what percentage X is of Y?
- Divide X by Y and multiply by 100. For example, 30 is what percent of 150? (30 / 150) × 100 = 20%. Use the 'X is what % of Y' tab.
- How is percentage change calculated?
- Percentage change = ((new value − old value) / |old value|) × 100. A positive result means an increase; negative means a decrease. For example, from 80 to 100: ((100 − 80) / 80) × 100 = 25% increase.
- What is the difference between percentage change and percentage difference?
- Percentage change has a clear direction: from an old value to a new value. Percentage difference (used when neither value is the 'original') uses the average of the two values as the denominator. This calculator uses percentage change.
- Can I calculate a percentage decrease?
- Yes. Use the '% Change' tab. If the 'From' value is larger than the 'To' value, the result will show a decrease: for example from 100 to 75 = 25% decrease.
- How do I work out a percentage increase?
- New value = old value × (1 + percentage / 100). For example, 200 increased by 15% = 200 × 1.15 = 230. Or use '% Change' to verify: from 200 to 230 = 15% increase.
- Why does the formula show absolute value for percentage change?
- The formula divides by |old value| (the absolute value) to handle negative starting values correctly. The sign of the result already indicates increase or decrease.
- Is there a history of calculations?
- Yes. The Recent Calculations panel below the inputs shows the last five computations across all modes, letting you quickly compare results.
- Is this calculator free?
- Completely free. No account required. All calculations run in your browser.
AlteredIdea vs alternatives
vs other online tools: Everything runs in your browser: private, instant, no account needed.
vs desktop apps: No install required. Works on any device.
vs paid tools: Completely free, unlimited use.