Mental Math Tricks
Work out bills, tips and totals in your head faster than you can find your phone's calculator. A handful of tricks covers most everyday sums.
~10 min4 lessonsTimed practice drill5-question quiz
01
Round, then fix it
Awkward numbers are easier when you round them to something friendly, do the sum, then correct.
Example · 47 + 38
- Round 38 up to 40: 47 + 40 = 87
- You added 2 too many, so take 2 off
- 85
Example · 6 × $19.99
- Treat it as 6 × $20 = $120
- Take off 6 × 1¢ = 6¢
- $119.94
Example · 83 − 29
- 83 − 30 = 53
- You took 1 too many, so add it back
- 54
02
Split hard multiplications
Break one number into tens and ones, multiply each part, then add.
Example · 7 × 48
- 7 × 40 = 280
- 7 × 8 = 56
- 280 + 56
- 336
Or halve and double: halve one number, double the other, and the answer doesn't change.
Example · 16 × 25
- Halve 16, double 25: 8 × 50
- Again: 4 × 100
- 400
03
Shortcuts for 5, 9, 11 and 25
| To multiply by… | Do this | Example |
|---|---|---|
| 5 | × 10, then halve | 5 × 86 → 860 ÷ 2 = 430 |
| 9 | × 10, then subtract the number | 9 × 23 → 230 − 23 = 207 |
| 11 (2-digit) | Split the digits, put their sum in the middle | 11 × 35 → 3_(3+5)_5 = 385 |
| 25 | × 100, then divide by 4 | 25 × 36 → 3,600 ÷ 4 = 900 |
11s with a carry: 11 × 78 → 7 _(7+8=15)_ 8. Write the 5 in the middle and carry the 1 onto the 7: 858.
04
Squaring numbers ending in 5
Take the first digit(s), multiply by the next number up, and stick 25 on the end.
Example · 35²
- 3 × 4 = 12
- Add 25 to the end
- 1,225
Example · 85²
- 8 × 9 = 72
- Add 25 to the end
- 7,225
Try it · 60-second drill
Press Start