Mental math tricks for percentages, tips, times tables and change

The most useful mental math tricks turn one hard sum into two easy ones. Find 10% by moving the decimal point, then halve it for 5%. Multiply by 5 by halving and adding a zero. Multiply by 9 by taking ten lots and removing one. Round awkward numbers, then adjust. Practise on separate days.

Here are the ten I use most, each with a worked example.

Key points

• Percentages are mostly 10% and 5% stuck together. 15% is 10% plus half of it.
• Multiplying by 5, 9, 11 and 25 each has a shortcut that turns it into easy steps.
• Squares near 50 follow one simple rule: 2,500, plus or minus 100 for each step away, plus the step squared.
• Round and adjust, and counting up for change, are the two tricks I use every week.
• Tricks stick when you test yourself and spread the practice out, rather than rereading a list.

What are the quickest mental math tricks for percentages?

Start with 10%: move the decimal point one place left, so 10% of 64 is 6.4. Half of that is 5%, which is 3.2. Add them and 15% of 64 is 9.6. For 20%, double the 10%. Nearly every everyday percentage, from tips to discounts, is built from these pieces.

1. Ten percent, then build. 15% of a bill of 48 is 4.8 plus 2.4, so 7.2. A 20% tip on the same bill is 4.8 doubled, so 9.6. For 25%, halve twice: 48, then 24, then 12.

2. Swap the numbers. A percentage works both ways round, so 8% of 50 is the same as 50% of 8, which is 4. That's because both are 8 times 50, divided by 100. Use it whenever one of the two numbers is friendly. 4% of 75 becomes 75% of 4, which is 3.

How do you multiply by 5, 9 and 11 in your head?

Each one has a shortcut. To multiply by 5, multiply by 10 and halve. To multiply by 9, multiply by 10 and take one lot away. To multiply a two digit number by 11, write its two digits apart and put their sum in the middle, carrying if the sum is 10 or more.

3. Times 5. 68 × 5: ten lots is 680, half is 340. For an odd number, 47 × 5 is 470 halved, so 235.

4. Times 9. 47 × 9: ten lots is 470, minus one lot of 47 is 423. The same idea works for 99: 99 × 23 is 2,300 minus 23, so 2,277.

5. Times 11. 53 × 11: the digits 5 and 3, with 5 plus 3 in the middle, give 583. When the middle sum is 10 or more, carry the 1: 78 × 11 is 7, then 15, then 8, so 858.

6. Times 25, by doubling and halving. Halve one number and double the other, and the answer doesn't change. 16 × 25 becomes 8 × 50, then 4 × 100, so 400. Or divide by 4 and add two zeros: 36 × 25 is 9 with two zeros, 900.

How do you square numbers near 50?

Start from 2,500, which is 50 squared. For every step above 50, add 100; for every step below, take 100 away. Then add the square of the step. So 53 squared is 2,500 plus 300 plus 9, which is 2,809. And 47 squared is 2,500 minus 300 plus 9, so 2,209.

7. Squares near 50. It works because 53 is 50 plus 3, and squaring that gives 2,500, then twice 50 times 3, which is 300, then 3 times 3. Try 56: 2,500 plus 600 plus 36 is 3,136. Past about 8 steps away the last part gets heavy, so I'd use it from 42 to 58.

How do round and adjust and counting up for change work?

Round and adjust means you swap an awkward number for a round one, do the easy sum, then fix the difference. Counting up for change means you start at the price and count up to the amount paid, in friendly jumps, instead of subtracting. Both avoid carrying digits in your head, which is where most slips happen.

8. Round and adjust. 298 plus 467: call it 300 plus 467, which is 767, then take back the 2 you added, so 765. Multiplying works too: 6 × 199 is 6 × 200, which is 1,200, minus 6, so 1,194.

9. Count up for change. You pay 20 for something that costs 13.65. Count up: 13.65 to 14 is 0.35, then 14 to 20 is 6. The change is 6.35. I find it far easier than subtracting from 20.

10. Add from the left. 347 plus 286: hundreds first, 300 plus 200 is 500. Tens, 40 plus 80 is 120. Units, 7 plus 6 is 13. Then 500, 620, 633. Going left to right means you say the big part of the answer first, which is the part that matters most if you're estimating.

Which trick should I use when?

Pick the trick by the shape of the numbers, not by habit. If a number sits just below a round one, round and adjust. If one factor is 5, 25 or 11, use its shortcut. If it's a percentage, build from 10%. If you're giving change, count up. This table puts all ten side by side.

Trick	Use it for	Worked example
10% then build	Tips, discounts, 15%, 20%, 25%	15% of 64 is 6.4 plus 3.2, so 9.6
Swap the numbers	Percentages with one friendly number	8% of 50 is 50% of 8, so 4
Times 5	Any number times 5	68 × 5 is 680 halved, so 340
Times 9 or 99	Any number times 9 or 99	47 × 9 is 470 minus 47, so 423
Times 11	Two digit numbers times 11	53 × 11 is 5, 8, 3, so 583
Double and halve	Times 25, 50, or even pairs	16 × 25 is 4 × 100, so 400
Squares near 50	42 to 58 squared	53 squared is 2,500 plus 300 plus 9
Round and adjust	Numbers ending in 7, 8 or 9	298 plus 467 is 767 minus 2
Count up for change	Giving or checking change	20 minus 13.65 is 0.35 plus 6
Add from the left	Quick totals and estimates	347 plus 286 is 500, 620, 633

Why does practice with spaced review work?

Testing yourself beats rereading, and spreading practice over days beats cramming it into one sitting. Roediger and Karpicke found in 2006 that students who took recall tests remembered much more a week later than students who restudied, and a large review by Cepeda and colleagues found spaced sessions improve retention.

The details are worth knowing. In Roediger and Karpicke's study, published in Psychological Science, restudying actually won on a test five minutes later, and it made students more confident. Testing only pulled ahead on the delayed tests, at two days and a week. That's the trap: rereading a list of tricks feels like learning.

Cepeda and colleagues, in Psychological Bulletin, pooled 839 assessments from 317 experiments. Their main finding was that the gap between sessions that gives the most retention grows with how long you want to remember. And in 2013, Dunlosky and colleagues rated practice testing and distributed practice as the two high utility study techniques in their review of ten.

One caveat, and I'd rather say it than hide it: most of this research used words and facts, not arithmetic shortcuts. I think it carries over, since a trick is something you have to recall under pressure, but that's my reading, not a study's finding.

In practice: try a trick on five sums today, again tomorrow, again in three days, and again in a week. Say the answer before you check it.

How I built CalcFit around this

CalcFit turns these tricks into timed drills with the spacing done for you. I built it for Android. It starts with a short placement test, then builds a weak spot plan from the skills you're slowest or least accurate at. A missed problem type comes back within a few days, then again a week later.

Each skill keeps its own level, so it gets harder as you improve and eases off if you slip. After each set you see your accuracy, your average speed and the questions you missed. It works offline, needs no account, and keeps your history on your phone. It's on Google Play. More on the CalcFit page and in what is CalcFit. If you like the same kind of logic with letters, try my guide on how to solve cryptograms.

Frequently asked questions

What is the easiest mental math trick to learn first?

Ten percent. Move the decimal point one place to the left and you have it. From there, halving gives 5%, doubling gives 20%, and adding 10% and 5% gives 15%. It covers tips, discounts and sale prices, which are the sums most people meet every week.

How do you multiply by 11 in your head?

For a two digit number, split its digits and put their sum in the middle. 53 × 11 is 5, then 8, then 3, so 583. If the middle sum is 10 or more, carry the 1 to the left digit: 78 × 11 is 858.

How long does it take to get faster at mental math?

It depends on where you start and how often you practise. Short, regular sessions spread over days work better than long ones, according to research on spaced practice. I'd give any new trick a week of five minute sessions before judging whether it has stuck.

Is using tricks cheating?

No. Every trick here is ordinary arithmetic arranged into easier steps. Doubling and halving, rounding and adjusting, and building from 10% are how many quick counters already think. The trick only fails if you use it without checking whether the answer is roughly right.

Do mental math tricks help with aptitude tests?

They help with the arithmetic inside numerical reasoning questions, especially percentages and estimates under time pressure. They won't teach you to read charts or set up a problem. Practise both, and time yourself, because speed is part of what those tests measure.

How I wrote this

On 4 November 2026 I read the PubMed records for Roediger and Karpicke's 2006 testing study, Cepeda and colleagues' 2006 spacing review and Dunlosky and colleagues' 2013 review of study techniques. I checked every worked example by hand. I checked search volumes in Google Keyword Planner for the United States. The CalcFit details come from its Google Play listing and the app I built.

By John Osakwe, founder of Etest Tech Hub. I have built apps and websites since 2015, including Etest CBT, FizzFree and Pawlarm.