10 Mental Math Tricks That Will Make You Look Like a Genius

Being able to calculate quickly in your head is not just a party trick — it is a genuinely useful skill that saves time, builds number sense, and can even help you make better financial decisions. The good news is that mental math is not about being a genius. It is about knowing a handful of techniques that make hard problems easy.

Multiplying by 11

Here is one of the easiest and most impressive tricks. To multiply any two-digit number by 11, add the two digits together and put the sum between them. For example, 35 times 11: 3+5=8, so the answer is 385. If the sum is 10 or greater, carry the 1 to the first digit. So 87 times 11: 8+7=15, carry the 1 to get 957. This works for any two-digit number and takes about two seconds once you practice it a few times.

Squaring Numbers Ending in 5

Any number ending in 5 has a neat trick for squaring. Take the digit before the 5, multiply it by the next number, and append 25. So 35 squared: 3 times 4 is 12, so the answer is 1225. For 75 squared: 7 times 8 is 56, so the answer is 5625. This works because of the algebraic identity (10n+5) squared = 100n(n+1) + 25, but you do not need to know the algebra to use the trick.

The 9 Times Table Finger Trick

Hold your hands in front of you with fingers spread. To multiply 9 by any number from 1 to 10, fold down the finger corresponding to that number. The fingers to the left of the folded finger give you the tens digit, and the fingers to the right give you the ones digit. So for 9 times 3, fold down your third finger: you have 2 fingers to the left and 7 to the right, giving you 27. Kids love this one, and adults find it surprisingly hard to forget once they learn it.

Percentage Shortcuts

Calculating percentages mentally is one of the most practically useful skills you can develop. The key insight is that percentages are just fractions in disguise. To find 10% of any number, move the decimal point one place to the left. To find 20%, double that. To find 5%, halve the 10% value. To find 15%, add the 10% and 5% values together.

Here is a real example: you are at a restaurant and the bill is $47.80. A 15% tip: 10% is $4.78, 5% is half of that ($2.39), so 15% is $4.78 + $2.39 = $7.17. A 20% tip is just double the 10%: $9.56. You can do this in under five seconds with practice, and it means you never need to pull out your phone to calculate a tip.

Subtracting from 1000

To subtract any number from 1000, subtract each digit from 9, except the last digit which you subtract from 10. So 1000 - 647: 9-6=3, 9-4=5, 10-7=3, giving you 353. This is incredibly useful in mental arithmetic and forms the basis of many other techniques. The same principle works for subtracting from 100, 10000, or any power of 10.

Dividing by 5

Dividing any number by 5 is the same as multiplying by 2 and dividing by 10. So 680 divided by 5: double 680 to get 1360, then divide by 10 to get 136. For odd numbers or numbers ending in 5, first multiply by 2, then move the decimal point. So 235 divided by 5: 235 times 2 is 470, divided by 10 is 47. This is faster than long division for most people and works for numbers of any size.

Building Speed Through Practice

The way to get fast at mental math is not to memorize hundreds of tricks — it is to practice a few core techniques until they become automatic. Start with the ones above: multiplying by 11, squaring numbers ending in 5, calculating percentages, and the 1000 subtraction trick. Practice them for five minutes a day with random numbers. Within a week, you will be noticeably faster. Within a month, you will be solving problems that used to require a calculator.