How to Get Better at Mental Math (5 Tricks)
Mental math isn’t about being born with a “math brain.” It’s about knowing a few patterns that turn hard arithmetic into easy arithmetic.
Quick answer
Break numbers apart, round and adjust, use the multiply-by-5 trick (×10 then ÷2), use the multiply-by-11 trick (digits + their sum), and learn percentages via 10% as a base.
1. Break numbers apart
To add 47 + 38: think 40 + 30 = 70, 7 + 8 = 15. Sum: 85. Faster than column arithmetic.
2. Round and adjust
To compute 98 × 5: round to 100 × 5 = 500. Subtract 2 × 5 = 10. Answer: 490.
3. Multiply by 5 = ×10 ÷ 2
34 × 5 = 340 ÷ 2 = 170. Faster than long multiplication.
4. Multiply by 11 (two-digit number)
For 23 × 11: take the digits (2 and 3), put their sum in the middle: 2 [2+3] 3 = 253. Works for 12 × 11 = 132 too (1 [1+2] 2). If the middle is ≥ 10, carry the 1 to the left.
5. Use 10% as a base for percentages
10% of any number is shifting the decimal: 10% of 250 = 25. To get 20%, double it (50). 5%, halve it (12.5). 15%, add 10% + 5% = 25 + 12.5 = 37.5. Almost any percentage can be built from 10%.
Bonus: square numbers near a base
To square a number near 50: square (n – 50), add to 25 hundreds. 53² = (53-50)² + 25 × 100 + 2 × 50 × (53-50) = 9 + 2500 + 300 = 2809.
Common mistakes
- Trying to do columns in your head instead of breaking numbers apart.
- Memorizing facts instead of understanding patterns.
- Not practicing — mental math is a skill that needs daily reps.