You don't need a calculator for most everyday percentages. With one trick, finding 10%, and a few building blocks, you can work out tips, discounts and commissions in a few seconds.
Start with 10%
To find 10% of any number, move the decimal point one place to the left. 10% of 80 is 8. 10% of 240 is 24. 10% of 1,350 is 135.
From there, halve it for 5%, and move the decimal one more place for 1%.
Build the percentage you need
10%
80 ÷ 10equals8
5% (half of 10%)
8 ÷ 2equals4
15%
8 + 4equals12
10%
240 ÷ 10equals24
5%
24 ÷ 2equals12
2.5%
12 ÷ 2equals6
17.5%
24 + 12 + 6equals42
Useful shortcuts
- 25% is a quarter: halve, then halve again. 25% of 200 is 50.
- 50% is half, and 20% is a fifth: 10% doubled.
- Percentages can be swapped: 8% of 50 is the same as 50% of 8, which is 4.
- For a discount, find the percentage and subtract it. 30% off 70: 10% is 7, so 30% is 21, and the price is 49.
Percentages as fractions
Some percentages are easier as fractions. 15% is 3/20, 12.5% is 1/8, 20% is 1/5 and 75% is 3/4. If a number divides neatly by the bottom of the fraction, the fraction is often the fastest route.
Tips, sales tax and commission
The same building blocks cover most everyday situations. For an 18% tip on a $65 bill, 10% is 6.50, 5% is 3.25, and 1% is 0.65, so 3% is 1.95. Added together that is 11.70.
For a 12% commission on a $2,500 sale, 10% is 250 and 1% is 25, so 2% is 50 and the total is 300. The pattern is always the same: find 10%, then build up or down from there.
Tip: For price changes, remember that the percentage always applies to the starting number. A 20% rise followed by a 20% fall does not bring you back to where you began.