How to Calculate Percentages in Your Head
Easy mental tricks for working out tips, discounts and percentage changes without a calculator, using 10%, 5% and 1% as building blocks.
You do not need a calculator for most everyday percentages. With three building blocks, 10%, 5% and 1%, you can work out tips, discounts and price changes in a few seconds.
Building block 1: 10%
Move the decimal point one place to the left. 10% of 340 is 34. 10% of 72.50 is 7.25.
Building block 2: 5%
Halve the 10% answer. 5% of 340 is 17.
Building block 3: 1%
Move the decimal point two places to the left. 1% of 340 is 3.4.
Combining them
| You want | Build it from | On 340 |
|---|---|---|
| 15% | 10% + 5% | 34 + 17 = 51 |
| 20% | 10% × 2 | 68 |
| 25% | Divide by 4 | 85 |
| 18% | 20% − 2 × 1% | 68 − 6.8 = 61.2 |
| 30% | 10% × 3 | 102 |
The swap trick
X% of Y is always the same as Y% of X. 8% of 50 sounds awkward, but 50% of 8 is obviously 4. Swap whenever one way round is easier.
Prices after a discount
Instead of finding the discount and subtracting, find what is left. 30% off means you pay 70%. On a 60 item, 70% is 7 × 6 = 42.
Percentage change
Divide the change by the starting value. A bill that went from 80 to 92 rose by 12. 12 out of 80 is the same as 15 out of 100, so it rose 15%.
Tip Round first to estimate, then refine. 18% of 47.60 is close to 20% of 48, which is about 9.6, so the exact answer, 8.57, should be a little lower.
Try it yourself: Percentage CalculatorPercent of a number, what percent one number is of another, and percent change, all at onceOpen →Common questions
How do I work out a 15% tip quickly?
Take 10%, then add half of that. On 46, that is 4.60 + 2.30 = 6.90.
How do I find what percent one number is of another in my head?
Turn it into a fraction you know. 18 out of 24 is 3/4, which is 75%.
Why does 50% off then 50% off not make it free?
The second discount is taken from the already reduced price, so you pay 25% of the original.