10 Mental Math Tricks Anyone Can Learn
Ten mental math tricks with worked examples: multiply by 5, 9, 11 and 25, square numbers ending in 5, subtract from 1000, and more. Each one takes minutes to learn.
On this page
- 1. Multiply by 5: times 10, then halve
- 2. Multiply by 9: times 10, then take one away
- 3. Multiply a two-digit number by 11
- 4. Multiply by 25: times 100, then divide by 4
- 5. Double and halve
- 6. Square a number ending in 5
- 7. Multiply two numbers near 100
- 8. Subtract from 1000: all from 9, the last from 10
- 9. Round and adjust for addition
- 10. Swap a percentage around
- How to make the tricks stick
- The bottom line
Some calculations look hard but have a shortcut that makes them almost instant. Here are ten of the most useful, each with a worked example and a few problems to try. You don’t need to learn them all today: pick one, use it until it feels natural, then come back for the next.
1. Multiply by 5: times 10, then halve
5 is half of 10. So multiply by 10, which just adds a zero, then halve.
5 × 86
- 86 × 10 = 860
- 860 ÷ 2 = 430
2. Multiply by 9: times 10, then take one away
9 is 10 − 1. Multiply by 10, then subtract the number once.
9 × 47
- 47 × 10 = 470
- 470 − 47 = 423
3. Multiply a two-digit number by 11
Split the digits apart and put their sum in the middle.
11 × 36
- 3 + 6 = 9
- put 9 between 3 and 6: 396
If the sum is 10 or more, carry the 1 into the first digit:
11 × 78
- 7 + 8 = 15
- write 5 in the middle, carry 1: 7 + 1 = 8
- 858
4. Multiply by 25: times 100, then divide by 4
25 is a quarter of 100. Multiply by 100, then halve twice.
25 × 36
- 36 × 100 = 3600
- 3600 ÷ 4 = 900
Or the other way round: 36 ÷ 4 = 9, then 9 × 100 = 900.
5. Double and halve
Halving one number and doubling the other doesn’t change the answer, and it often turns a hard problem into an easy one.
16 × 35
- halve 16, double 35: 8 × 70
- 8 × 70 = 560
Keep going while it helps: 14 × 45 becomes 7 × 90 = 630.
6. Square a number ending in 5
Take the tens digit, multiply it by the next number up, then write 25 after it.
65²
- 6 × 7 = 42
- write 25 after it: 4225
It works because 65² = 60 × 70 + 5². The same pattern gives 35² = 1225 and 85² = 7225.
7. Multiply two numbers near 100
When both numbers are just under 100, work with how far each is from 100.
96 × 97
- 96 is 4 under, 97 is 3 under
- 96 − 3 = 93 (or 97 − 4)
- 4 × 3 = 12
- write them together: 9312
8. Subtract from 1000: all from 9, the last from 10
To take any three-digit number from 1000, take each digit from 9 except the last, which you take from 10.
1000 − 368
- 9 − 3 = 6
- 9 − 6 = 3
- 10 − 8 = 2
- 632
Handy for change: if something costs 3.68 and you pay with 10, you get 6.32 back.
9. Round and adjust for addition
Turn an awkward number into a round one, add, then put back the difference.
347 + 198
- 198 is 200 − 2
- 347 + 200 = 547
- 547 − 2 = 545
10. Swap a percentage around
x% of y is the same as y% of x. Pick whichever is easier.
8% of 50
- same as 50% of 8
- half of 8 is 4
More on this in our guide to percentages in your head.
Try it
- 5 × 68
Answer
340 - 11 × 54
Answer
594 - 25 × 44
Answer
1100 - 45²
Answer
2025 - 98 × 94
Answer
9212 - 1000 − 579
Answer
421
How to make the tricks stick
A trick you have read about is not yet a trick you can use. Three things make the difference:
- Practise each trick on its own first, on ten to twenty problems, until you don’t have to think about the steps.
- Then mix them. Real problems don’t tell you which trick to use, so practise choosing.
- Review after a few days. A trick you used once fades quickly; one you’ve come back to stays.
The bottom line
None of these tricks is hard, and each one saves real time once it’s automatic. Start with multiplying by 5 and 9, which you’ll use every day, then add the others one by one.
Every trick on this list is a lesson in Mathlite, with a worked example, a guided try and practice that adapts to your level, plus short sprints that bring old tricks back before you forget them.
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
What is the easiest mental math trick to start with?
Multiplying by 5 is a good first trick: multiply by 10 and halve. It works for any number, takes seconds to learn and is useful every day.
Do these tricks work with bigger numbers?
Most of them do. Multiplying by 5, 9 and 25 works for any number, and the 11 trick extends to longer numbers by adding neighbouring digits. Squaring numbers ending in 5 works for 105, 115 and beyond too.
How do I remember all these tricks?
Learn one at a time and use it on ten to twenty problems before moving on. Coming back to it a few days later makes it stick far better than learning all ten at once.
Are mental math tricks the same as Vedic math?
Some of them overlap. Tricks like subtracting from 1000 or multiplying numbers near 100 appear in Vedic mathematics, but they are ordinary arithmetic shortcuts that work for anyone.