How to Calculate Percentages
A percentage is simply a fraction out of 100 — the word itself comes from the Latin per centum, meaning “by the hundred.” Percentages show up everywhere: store discounts, tips, exam grades, interest rates, and statistics. There are three calculations most people need, and this tool handles all of them.
The Three Percentage Formulas
- X% of Y: (X ÷ 100) × Y. Example: 20% of 150 = 0.20 × 150 = 30.
- X as a percent of Y: (X ÷ Y) × 100. Example: 30 is (30 ÷ 150) × 100 = 20% of 150.
- Percentage change: ((New − Old) ÷ Old) × 100. A price rising from $100 to $125 is a 25% increase.
Everyday Uses
Shopping sales (“30% off”), restaurant tips, grade calculations, and salary raises all come down to these same three formulas. Once you see the pattern, you can estimate most percentages mentally: 10% of anything is just moving the decimal point one place left.
Percent vs. Percentage Points
One common trap: if an interest rate rises from 5% to 7%, that is a 2 percentage point increase — but a 40% percent increase relative to the original 5%. News headlines mix these up constantly. When comparing two percentages, always check which one the writer means.
Need quick arithmetic for the steps above? Try our four-function calculator.