How to calculate a percentage
To calculate a percentage, divide the part by the whole and multiply by 100. For a percent of a number, convert the percent to a decimal (divide by 100) and multiply. Open the percentage calculator.
The three percentage calculations
1. Find a percentage of a number (X% of Y)
result = (X ÷ 100) × Y. Example: 15% of 200 = 0.15 × 200 = 30.
2. Find what percentage one number is of another
percentage = (part ÷ whole) × 100. Example: 25 of 200 = (25 ÷ 200) × 100 = 12.5%.
3. Calculate percentage change
change% = ((new − original) ÷ original) × 100. Example: 80 to 100 = ((100 − 80) ÷ 80) × 100 = 25% increase.
Three worked examples
- 20% discount on $150: 0.20 × 150 = $30 off, sale price $120.
- 42 out of 60 on a test: (42 ÷ 60) × 100 = 70%.
- Salary $50,000 to $54,000: ((54,000 − 50,000) ÷ 50,000) × 100 = 8% increase.
Mental arithmetic techniques
- Find 10% by moving the decimal point one place left.
- Build up: 20% is double 10%, 5% is half of 10%, 15% is 10% + 5%.
- Swap the percentage and the number: X% of Y = Y% of X.
- A 50% increase then a 50% decrease leaves you at 75%, not 100%.
Common mistakes
- Confusing the base — the whole is the denominator in “what percent” questions.
- Percentage change is undefined when the original is 0.
- A move from 10% to 12% is 2 percentage points but a 20% relative increase.
- Stacked discounts multiply: 20% then 10% off is 28% off, not 30%.
Frequently asked questions
Frequently asked questions
- How do you calculate a percentage of a number?
- Divide the percentage by 100 and multiply by the number. For 15% of 200: (15 ÷ 100) × 200 = 30.
- How do I calculate what percentage one number is of another?
- Divide the part by the whole and multiply by 100. For 25 out of 200: (25 ÷ 200) × 100 = 12.5%.
- How do I calculate percentage change?
- Subtract the original from the new value, divide by the original, and multiply by 100: ((new − original) ÷ original) × 100. A rise from 80 to 100 is 25%.
- Why does a 50% increase followed by a 50% decrease not return the original?
- Each percentage applies to a different base. 100 + 50% = 150; 150 − 50% = 75. Equal up and down percentages do not cancel.
- What is the difference between percent and percentage points?
- A change from 10% to 12% is a 2 percentage-point increase but a 20% relative increase. Use percentage points for rates, percent for relative growth.
- What is the fastest way to calculate a tip mentally?
- Find 10% by moving the decimal one place left, then scale. For 18% on $45: 10% is $4.50, 20% is $9.00, so 18% is about $8.10.
Part of the Math Calculators cluster. Related groups: Finance, Statistics, Education, Date & Time.
Reviewed by CalcFlow Editorial Team. Last reviewed 2026-09.