How to Work Out Percentages in Your Head (and Tips and Discounts)
Find 10%, 5%, 15%, 20% and 25% of any number in seconds, work out tips and sale prices, and use the swap trick that makes awkward percentages easy.
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Percentages turn up everywhere: tips, sales, interest, taxes, test scores. The good news is that you can work out almost any of them in your head from one starting point, 10%.
Start with 10%
10% means a tenth. To find it, move the decimal point one place to the left.
10% of 260
- move the point one place left
- 26
10% of 74.50 is 7.45. 10% of 9 is 0.9. Once you have 10%, most other percentages are a step away.
Build the others from 10%
| Percentage | How to get it | Of 80 |
|---|---|---|
| 10% | move the point one place | 8 |
| 5% | half of 10% | 4 |
| 20% | double 10% | 16 |
| 15% | 10% + 5% | 12 |
| 30% | 3 × 10% | 24 |
| 1% | move the point two places | 0.8 |
| 25% | a quarter: halve twice | 20 |
| 50% | half | 40 |
15% of 46
- 10% = 4.6
- 5% = 2.3
- 4.6 + 2.3 = 6.9
35% of 120
- 10% = 12
- 30% = 36
- 5% = 6
- 36 + 6 = 42
Tips in seconds
The common tip sizes all come from 10%:
- 10%: move the point. On 38.00, that’s 3.80.
- 15%: 10% plus half of it. On 38.00, 3.80 + 1.90 = 5.70.
- 20%: double 10%. On 38.00, 7.60.
- 18%: 20% minus 2%. On 50.00, 10.00 − 1.00 = 9.00.
It’s fine to round the bill first. For a 20% tip on 47.80, round to 48: 10% is 4.80, double is 9.60.
Sale prices and discounts
When something is marked down, you usually want the price you pay, not the amount you save. So work out the part you pay:
30% off 80
- you pay 70%
- 10% of 80 = 8
- 7 × 8 = 56
25% off 64
- you pay 75%, three quarters
- a quarter of 64 is 16
- 64 − 16 = 48
Watch out for two discounts in a row: 20% off and then another 10% off is 28% off in total, not 30%, because the second discount comes off the already reduced price.
The swap trick
x% of y is always the same as y% of x. When a problem looks awkward, flip it.
16% of 25
- same as 25% of 16
- a quarter of 16 is 4
12% of 50
- same as 50% of 12
- half of 12 is 6
What percentage is it?
To find what percentage one number is of another, turn it into a fraction you know.
18 out of 24
- 18/24 = 3/4
- 3/4 = 75%
The fractions worth knowing by heart: 1/2 = 50%, 1/4 = 25%, 3/4 = 75%, 1/5 = 20%, 1/10 = 10%, 1/3 ≈ 33%, 2/3 ≈ 67%, 1/8 = 12.5%.
Try it
- 15% of 70
Answer
10.5 - 20% of 135
Answer
27 - 40% off 90 (price you pay)
Answer
54 - 4% of 75
Answer
3 - 21 out of 28 as a %
Answer
75%
The bottom line
Start from 10% and build: halve it for 5%, double it for 20%, add the two for 15%. For discounts, work out what you pay. And when a percentage looks awkward, swap it around. With a little practice, a 15% tip takes less time than opening a calculator app.
Mathlite’s percentage lessons cover 10%, building from 10%, quarters, swapping and % off, each with worked examples and practice that adapts to your level.
Practise this in Mathlite
Every trick in this article is a lesson in Mathlite, with worked examples and practice that adapts to you.
Frequently asked questions
How do you calculate 15% in your head?
Find 10% by moving the decimal point one place left, then add half of it, which is 5%. For 15% of 60: 10% is 6, 5% is 3, so 15% is 9.
How do you work out a 20% tip quickly?
Find 10% of the bill and double it. On a bill of 45, 10% is 4.50, so a 20% tip is 9.
How do I find the price after a discount?
Work out what you pay instead of what you save. With 30% off, you pay 70%: on 80, 10% is 8, so 70% is 56.
What is the percentage swap trick?
x% of y equals y% of x. So 16% of 25 is the same as 25% of 16, which is a quarter of 16, or 4.