Guide
8 Mental Math Tricks With Worked Examples
The fastest mental math tricks turn one hard calculation into two easy ones: round and then correct, or split a number into parts you already know. Below are eight methods with worked examples, followed by games where you can practise them against the clock.
Practise the tricks
1. Round and compensate
Round one number to something easy, do the calculation, then undo the rounding. It works for adding, subtracting and multiplying.
- 198 + 47 = 200 + 47 − 2 = 245
- 83 − 29 = 83 − 30 + 1 = 54
- 49 × 6 = 50 × 6 − 6 = 294
2. Add from left to right
On paper you add columns from the right, but in your head it is easier to start with the biggest part and keep a running total.
456 + 327: 400 + 300 = 700, then 50 + 20 = 70, then 6 + 7 = 13. Together that is 700 + 70 + 13 = 783.
Ghost Train practises this running total: numbers board and leave, and you keep the sum in your head.
3. Multiply by 5, 25 and 50
Multiplying by 5 is the same as multiplying by 10 and halving. The same idea covers 25 (multiply by 100, divide by 4) and 50 (multiply by 100, halve).
- 68 × 5 = 680 ÷ 2 = 340
- 36 × 25 = 3,600 ÷ 4 = 900
- 84 × 50 = 8,400 ÷ 2 = 4,200
4. Multiply by 9 and 11
To multiply by 9, multiply by 10 and subtract the number once: 37 × 9 = 370 − 37 = 333.
To multiply a two-digit number by 11, add its digits and put the sum in the middle: 53 × 11 gives 5 + 3 = 8, so 583. If the digits add up to 10 or more, carry the 1: 78 × 11 gives 7 + 8 = 15, so 7 + 1 = 8 at the front, then 5, then 8, which makes 858.
5. Square numbers that end in 5
Multiply the tens digit by the next whole number up, then write 25 after the result.
- 35²: 3 × 4 = 12, so 1,225
- 65²: 6 × 7 = 42, so 4,225
6. Double and halve
When one factor is even, halve it and double the other. The product does not change, and the new pair is often easier.
- 16 × 35 = 8 × 70 = 560
- 14 × 45 = 7 × 90 = 630
7. Build percentages from 10%
Move the decimal point one place to the left to get 10%, then combine: 15% of 80 is 10% (8) plus 5% (4), which is 12.
Percentages can also be swapped: 8% of 50 is the same as 50% of 8, which is 4.
8. Check with a quick estimate
Before you trust an answer, round both numbers and see whether the result is in the right range. 412 × 19 is about 400 × 20 = 8,000, so 7,828 is plausible and 78,280 has an extra digit.
How to make the tricks automatic
Knowing a method is not the same as using it under time pressure. Practise one trick at a time in short, timed rounds until you reach for it without thinking, then mix them.
Bubble Rush asks for quick answers to single equations, Equation Forge makes you build a target from four numbers and Reflex Duel has you compare two expressions at speed. The mental math games page lists them all.
Frequently Asked Questions
Which mental math trick should I learn first?
Round and compensate. It works for addition, subtraction and multiplication, and several of the other methods build on the same idea.
How do I get faster at mental math?
Learn one method at a time, practise it in short timed rounds until it becomes automatic, then mix methods. Short, regular sessions are easier to fit into a day than occasional long ones. If times tables slow you down, Grid Racer drills multiplication and division facts against three rivals.
Do these tricks work with bigger numbers?
Most of them do. Rounding and compensating works for 1,998 + 467 just as for 198 + 47, and halving after multiplying by 10 works for any number multiplied by 5. The limit is how many intermediate results you can hold in mind, so write them down at first.
Is being fast at mental math a sign of intelligence?
Speed at arithmetic mostly reflects practice and familiarity with number facts. It is a useful skill, but a fast score in a game is not an IQ measure.
Updated 4 November 2026