Guide
How Percentages Work (and How to Calculate Them)
Understand what a percentage is, the three core percentage calculations, and how to avoid common percentage mistakes.
A percentage expresses a number as a fraction of 100. A percentage calculator handles the three calculations people use most: finding a percentage of a number, finding what percentage one number is of another, and measuring percentage change.
What a percentage is
The word "percent" means "per hundred". 25% means 25 out of every 100, or equivalently the decimal 0.25. Converting between percentages and decimals is the key skill: divide by 100 to go from percent to decimal, multiply by 100 to go back.
The three core calculations
- X% of Y = (X ÷ 100) × Y. Example: 20% of 50 = 0.20 × 50 = 10.
- X is what % of Y = (X ÷ Y) × 100. Example: 10 is what % of 50 = (10 ÷ 50) × 100 = 20%.
- Percentage change = ((New − Old) ÷ |Old|) × 100. Example: 40 to 50 = ((50 − 40) ÷ 40) × 100 = 25% increase.
How to use a percentage calculator
- Identify which of the three calculations you need.
- Enter the two known values.
- Read the result, which the tool computes instantly.
- Double-check by estimating: 10% is easy (move the decimal one place), so build from there.
Examples
A $60 item with 25% off: 25% of 60 = 15, so the sale price is $45. A test score of 18 out of 20: (18 ÷ 20) × 100 = 90%. A salary rise from $40,000 to $46,000: ((46,000 − 40,000) ÷ 40,000) × 100 = 15% increase.
When a percentage calculator is useful
- Working out discounts, tax, and tips while shopping or dining.
- Calculating test scores, grades, or survey results.
- Measuring growth or decline in business metrics.
- Comparing proportions in reports and presentations.
Common mistakes
Confusing percentage points with percent change is the classic error: a rate rising from 10% to 12% is a 2 percentage-point increase, but a 20% relative increase. Always confirm which comparison you need.
Put it into practice
Use the Percentage Calculator right now — free, in your browser, no sign-up required.
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Related free tools
More free online utilities that pair well with this one — same-category tools for the same task, plus handy extras from other categories.