Mental Maths Tricks: Calculate Faster in Your Head

You do not need a calculator for most everyday sums. With a few simple tricks, you can add, subtract, multiply and work out percentages in your head — faster than reaching for your phone, and with far more confidence.

1. Add by Rounding to a Friendly Number

Round numbers are easy to add, so change the sum into one that uses them. Round one number up to the nearest ten, add, then take away what you added.

The same idea works with bigger numbers. For 136 + 258, add the hundreds first, then the rest: 136 + 200 = 336, then 336 + 58 = 394.

2. Subtract by Counting Up

Subtraction feels hard when you borrow across digits. Instead, count up from the smaller number to the bigger one — the steps you take are the answer. This is exactly how shopkeepers work out change.

Try it with 250 − 178: 178 to 180 is 2, 180 to 250 is 70, so the answer is 72.

3. Double and Halve for Multiplication

When a multiplication looks awkward, halve one number and double the other. The answer stays the same, but the sum gets easier.

For 14 × 15, halve 14 to 7 and double 15 to 30, giving 7 × 30 = 210.

4. Multiply by 5: Half of Times Ten

Multiplying by 5 is the same as multiplying by 10 and halving the result. Times ten is just adding a zero, and halving is usually easy.

If the number is odd, the halving leaves a .5 that becomes a 5 in the units place. For 37 × 5: 37 × 10 = 370, half of 370 = 185.

5. Multiply by 9: Times Ten, Minus the Number

Nine is one less than ten, so multiply by ten and subtract the number once.

The same trick works for other numbers close to ten. For × 11, multiply by ten and add the number once instead — see the next trick.

6. Multiply by 11

For two-digit numbers, the quick way is: multiply by ten, then add the number itself.

There is also a neat digit trick for two-digit numbers. For 34 × 11, put the sum of the digits between them: 3 + 4 = 7, so the answer is 374. When the digits add to ten or more, carry the one: 58 × 11 gives 5 + 8 = 13, so the answer is 638 — the 3 sits in the middle and the extra 1 carries to the 5.

7. Quick Percentages

Almost every percentage question is built from a few easy pieces: 10%, 5%, 25% and 50%. Learn those four and you can build the rest.

• 50% is half. 50% of 240 = 120.
• 25% is a quarter. 25% of 240 = 60.
• 10% is moving the decimal point one place left. 10% of 240 = 24.
• 5% is half of 10%. 5% of 240 = 12.

Combine the pieces for anything else. A 15% tip on 240 is 10% plus 5%: 24 + 12 = 36. A 20% discount is simply two lots of 10%.

8. Estimate First, Then Check

Before you calculate exactly, round the numbers and work out a rough answer. If your exact answer is far from the estimate, you know something went wrong — this one habit catches most mistakes.

With money, three items costing about 45, 60 and 80 total roughly 185 — so a bill of 1,850 would be an obvious error.

9. Square Numbers Ending in 5

Any two-digit number ending in 5 has a shortcut. Multiply the tens digit by the next number up, then write 25 at the end.

It works every time: 65² is 6 × 7 = 42, giving 4,225. And 85² is 8 × 9 = 72, giving 7,225. Try 45² yourself — the answer is 2,025.

10. Break Big Numbers into Parts

Large numbers are easier when you split them into tens and units and handle each part separately.

It works for subtraction too. For 573 − 248, take away 200, then 40, then 8: 573 − 200 = 373, 373 − 40 = 333, 333 − 8 = 325.

Daily Practice Drills

Tricks only become fast with practice, but five minutes a day is enough. A little regular practice beats one long session, because the methods need to become automatic.

• Five minutes a day: pick one trick each day and run through ten sums with it. Rotate through the tricks across the week.
• Timed rounds: set a timer for two minutes and see how many sums you can finish. Try to beat yesterday’s score, not someone else’s.
• Practise with prices and bills: add up your shopping as you go, work out the 10% and 15% tip at a restaurant, or check the change you are given. Real money makes the practice stick.
• Estimate before you calculate: make it a habit in everyday life — rough answer first, exact answer second.

Maths games are an easy way to keep the daily habit going. Addition, times tables and percentage games give you quick rounds of practice every day, and a short game is much easier to fit in than a worksheet. You can play free brain games across 150 categories at Trending Brain Games, and for general play advice, see our Tips & Strategies page.