Mental math tricks let you calculate faster, check your work without a calculator, and feel more confident with numbers in everyday life. These techniques are used by the world’s fastest mental calculators — and once you learn them they become second nature.
Percentage Tricks
Finding 10% of Any Number
10% of 340 = 34 | 10% of 78 = 7.8 | 10% of 1,250 = 125
Just move the decimal point one place to the left. This is the foundation for all other percentage calculations.
Finding 15% (Restaurant Tip Trick)
15% of $48: 10% = $4.80, half of that = $2.40, total = $7.20
Find 10%, then find half of that (5%), then add them together. Perfect for calculating restaurant tips instantly.
Finding 20%
20% of $65: 10% = $6.50, double it = $13.00
Find 10% and double it. Useful for tips, discounts, and tax calculations.
Finding 25%
25% of 80 = 80 ÷ 4 = 20 | 25% of 200 = 50
25% is the same as one quarter. Just divide by 4.
Flip the Numbers (Commutative Trick)
8% of 25 = 25% of 8 = 2 | 4% of 75 = 75% of 4 = 3
Percentages are reversible — A% of B = B% of A. Choose whichever is easier to calculate.
Multiplication Shortcuts
Multiplying by 5
5 × 48: 48 ÷ 2 = 24, then × 10 = 240 | 5 × 73 = 73 × 10 ÷ 2 = 365
Multiply by 10 then divide by 2, or divide by 2 then multiply by 10. Whichever comes first in your head.
Multiplying by 9
9 × 7: 10 × 7 = 70, minus 7 = 63 | 9 × 23 = 230 − 23 = 207
Multiply by 10 and subtract the number. Works for any multiplication by 9.
Multiplying by 11 (Single Digits)
11 × 7 = 77 | 11 × 4 = 44 | 11 × 9 = 99
Any single digit times 11 is just that digit repeated.
Squaring Numbers Ending in 5
35² = 3 × 4 = 12, add 25 = 1,225 | 75² = 7 × 8 = 56, add 25 = 5,625
Take the tens digit, multiply it by the next number up, then append 25 to the result.
Addition and Subtraction Tricks
Round Up Then Adjust
47 + 38: think 47 + 40 = 87, then subtract 2 = 85
Round one number to the nearest 10, do the easy addition, then adjust. Much faster than column addition.
Subtract by Adding Up
83 − 47: 47 + 3 = 50, 50 + 33 = 83. Answer = 3 + 33 = 36
Count up from the smaller number to the larger. Easier than borrowing for many people.
Grouping to Make Tens
16 + 25 + 14: group 16 + 14 = 30, then 30 + 25 = 55
When adding multiple numbers, look for pairs that sum to 10, 20, or 100 first.
Practice These Tricks
Use our free daily challenge and practice mode to reinforce these techniques.
Play Today's Challenge →Frequently Asked Questions
How long does it take to get good at mental math?
Most people notice significant improvement in speed and confidence within 2 to 4 weeks of daily practice. The key is consistency — even 5 minutes a day of focused mental math practice is more effective than an hour once a week. Our daily challenge is designed around exactly this principle.
What is the most useful mental math skill to learn first?
Learning to quickly find 10% of any number is the single most useful skill to master first. Once you can find 10% instantly, you can calculate 5%, 15%, 20%, 30%, and almost any other percentage with simple additions and doublings. It is the foundation that all other percentage calculations build on.