To find X% of a number, divide X by 100 and multiply by the number. For example, 20% of 150 = 20 ÷ 100 × 150 = 30. That single idea — "per cent" means "per hundred" — solves almost every percentage problem you'll meet, from tips and sales tax to discounts and interest.
1. Percent of a number
Formula: (percent ÷ 100) × number.
- 15% of 200 = 0.15 × 200 = 30
- 8.25% sales tax on $50 = 0.0825 × 50 = $4.13
- A 20% tip on a $60 bill = 0.20 × 60 = $12
Try any values in the percentage calculator — it shows the formula too.
2. What percent is X of Y?
Formula: (part ÷ whole) × 100.
- 30 out of 150 = 30 ÷ 150 × 100 = 20%
- You saved $45 on a $300 item = 45 ÷ 300 × 100 = 15% off
Quick link: what percent is 30 of 200.
3. Percentage increase or decrease
Formula: (new − old) ÷ old × 100. A positive answer is an increase, negative is a decrease.
- Price rose from 80 to 100: (100 − 80) ÷ 80 × 100 = +25%
- Add 15% to 200: 200 × 1.15 = 230 — see 200 plus 15%
- Take 30% off 80: 80 × 0.70 = 56
Handy mental-math tricks
- 10% = move the decimal one place left (10% of 240 = 24).
- 1% = move it two places (1% of 240 = 2.40).
- 5% = half of 10%. 20% = double 10%.
- X% of Y = Y% of X — so 18% of 50 is the same as 50% of 18 = 9.
Where you use percentages with money
Percentages run through every money decision: VAT and sales tax, interest (APR and APY), investment returns, and your effective tax rate. Master the three formulas above and the rest is just plugging in numbers.
Frequently asked questions
- How do you calculate a percentage of a number? (percent ÷ 100) × number — e.g. 20% of 150 = 30.
- What percent is one number of another? (part ÷ whole) × 100 — e.g. 30 of 150 = 20%.
- How do you add a percentage to a number? Multiply by (1 + percent ÷ 100) — e.g. +15% means × 1.15.