Everyday percentage formulas with worked examples

Percent means “per hundred.” These three questions look similar but use different calculations. Identify the whole amount before entering numbers into a calculator.

Find a percentage of a number

Formula: part = percentage ÷ 100 × whole. For example, 15% of 240 is 0.15 × 240 = 36. Enter 15 and 240 in the percentage calculator.

Find what percentage one number is of another

Formula: percentage = part ÷ whole × 100. If 18 of 60 questions are complete, 18 ÷ 60 × 100 = 30%. The whole is 60, not 18. This site's simple calculator computes a percentage of a number, so use the formula for this reverse question.

Apply a discount

Formula: sale price = original price × (1 − discount ÷ 100). An $80 item at 25% off costs $80 × 0.75 = $60 before tax. Use the discount calculator for the price and savings.

Calculate a tip

Formula: tip = bill × tip rate ÷ 100. A $45 bill with an 18% tip gives $8.10. The total is $53.10. The tip calculator can also split the total among people. Decide whether your starting bill includes tax.

Quick reasonableness checks

• 10% is one tenth of the whole; 5% is half of 10%.
• A 20% discount should leave 80% of the original price.
• When comparing two values, write down which number is the whole before dividing.