How to use the percentage calculator
Pick the question that matches what you want to know and type your two numbers into that row. You do not need to press anything. The answer updates as you type, and the other rows are not affected.
The formulas, with real examples
1. Percent of a number
Formula: (percent ÷ 100) × number
You want to leave a 15% tip on a bill of 80. 0.15 × 80 = 12.
2. What percent one number is of another
Formula: (part ÷ whole) × 100
You got 42 out of 50 on a test. 42 ÷ 50 × 100 = 84%.
3. Percentage change
Formula: ((new - old) ÷ old) × 100
Your rent went from 1,200 to 1,290. (1,290 - 1,200) ÷ 1,200 × 100 = 7.5% increase. If the result is negative, it is a decrease.
4. Find the original number
Formula: part ÷ (percent ÷ 100)
You paid 24 in tax and you know the tax rate is 20%. 24 ÷ 0.20 = 120, so the price before tax was 120.
Quick mental math tricks
- 10%: move the decimal point one place left. 10% of 250 is 25.
- 5%: half of 10%. 5% of 250 is 12.5.
- 25%: divide by 4. 50%: divide by 2.
- Swap trick: x% of y is the same as y% of x. So 8% of 50 is the same as 50% of 8, which is 4.
Working out a sale price? The discount calculator handles stacked discounts and sales tax for you.