Why it works
Per cent means “per hundred”: 1% = 1/100 = 0.01. So 15% of a number is fifteen hundredths of it, i.e. number × 0.15. You can multiply and divide in either order.
Mental shortcuts
10%: move the decimal point one place left (10% of 2,340 = 234). 5% is half of 10%. 1% moves it two places. 20% is double 10%, 25% is a quarter, 50% is half. Build anything else from pieces: 15% = 10% + 5%.
Where you need it
Discount amounts, sales tax or VAT, tips, bank fees, budget shares. To get the final price after a discount or with a markup, see the “subtract a percentage” and “add a percentage” pages.