10 Percentage Tricks for Mental Maths (Free Checker)
Find 10% by moving the decimal, build 15% or 18% from it, swap 8% of 50 for 50% of 8, and more free shortcuts for tips, discounts and exams.
Published · 4 min read
The best percentage tricks all start from 10%: move the decimal point one place left (10% of 640 is 64), then halve, double or add to reach the percentage you need. Add the swap trick (8% of 50 equals 50% of 8, which is 4) and a few sanity checks, and most everyday percentages take seconds in your head. When you need an exact answer, the free percentage calculator checks your working instantly.
These tricks are handy at a shop counter, splitting a restaurant bill, or in aptitude tests and bank exams where calculators aren't allowed.
1. Find 10% by moving the decimal
10% of any number is that number with the decimal point moved one place left.
- 10% of 640 = 64
- 10% of 86 = 8.6
- 10% of ₹2,350 = ₹235
Similarly, 1% moves the decimal two places: 1% of 4,800 = 48. These two are the building blocks for everything else.
2. Build other percentages from 10%, 5% and 1%
| Want | Build it as | Example: of 86 |
|---|---|---|
| 5% | half of 10% | 4.3 |
| 15% | 10% + 5% | 8.6 + 4.3 = 12.9 |
| 20% | 10% × 2 | 17.2 |
| 18% | 20% − 2% | 17.2 − 1.72 = 15.48 |
| 25% | divide by 4 | 21.5 |
| 30% | 10% × 3 | 25.8 |
A 15% tip on an $86 bill: 8.6 plus half of it, 4.3, is $12.90. For 18% GST on ₹450: 10% is 45, 20% is 90, minus 2% (9) gives ₹81.
3. Use the swap trick
X% of Y always equals Y% of X, because both are X × Y ÷ 100. Flip the question when one side is easier:
- 8% of 50 = 50% of 8 = 4
- 4% of 75 = 75% of 4 = 3
- 12% of 25 = 25% of 12 = 3
- 35% of 80 = 80% of 35 = 28
This one wins a surprising number of exam questions.
4. Know the fraction friends
Some percentages are fractions you already know:
| Percent | Fraction |
|---|---|
| 50% | 1/2 |
| 33⅓% | 1/3 |
| 25% | 1/4 |
| 20% | 1/5 |
| 12.5% | 1/8 |
| 10% | 1/10 |
12.5% of 72 is 72 ÷ 8 = 9. 25% off ₹1,600 is ₹400 off, because 1,600 ÷ 4 = 400. For awkward fractions, the free fraction to decimal converter shows the percent too.
5. Split big percentages into easy parts
17.5% looks hard, but it's 10% + 5% + 2.5%. Of 240: 24 + 12 + 6 = 42.
Or work from 100% downwards. 95% of 780 is 780 minus 5% (39) = 741. 90% of anything is the number minus a tenth.
6. Turn "% off" into "% you pay"
A discount is easier to handle as what's left. 15% off means you pay 85%:
- 15% off ₹499: 10% is 49.90, 5% is 24.95, so the discount is 74.85 and you pay ₹424.15.
For stacked discounts, multiply the "what you pay" parts: 20% off then 10% off is 0.8 × 0.9 = 0.72, so you pay 72% of the price. That's 28% off, not 30%. The Black Friday discount math post has more on this.
7. Add tax in one step
To add a percentage, multiply by 1 plus it: 18% GST means × 1.18. ₹1,500 × 1.18: 1,500 + 10% (150) + 8% (120) = ₹1,770. Going backwards is different: to take 18% GST out of a price, divide by 1.18, as the GST guide explains.
8. Reverse a percentage change correctly
A 10% rise followed by a 10% fall doesn't bring you back: 1.1 × 0.9 = 0.99, so you end 1% down. A 50% fall needs a 100% rise to recover. Quick rule: after a fall of X%, you need a bigger rise than X% to get back.
9. Sanity-check your answer
Before trusting a mental answer, check it against something obvious:
- Is it less than the number? Any percentage under 100% of a positive number must be smaller than the number.
- Is it near the 10% mark? 18% of 450 should be a bit under double 45 (90). 81 passes.
- Did the decimal go the right way? 10% of 2,350 is 235, not 23.5 or 2,350.
- Does the reverse work? If 81 is 18% of 450, then 81 ÷ 0.18 should give 450.
10. Estimate, then refine
For exam options or quick decisions, round first. 33% of 900 is about a third: 300 (exactly 297). 66% of 450 is about two-thirds: 300 (exactly 297). If the options are far apart, the estimate is enough. If they're close, refine with the 10% and 1% building blocks.
Practise the percentage tricks
The fastest way to get good is to guess first, then check. Open the percentage formulas guide for the three core formulas, and for restaurant tips try the free tip calculator, which also splits the bill.
Check your answer now
Make your guess, then type the numbers into the free percentage calculator to see the exact answer. It's free, needs no sign-up and works on your phone, so it's always there when the mental maths gets close.
Frequently asked questions
- What is the easiest way to calculate a percentage in your head?
- Find 10% by moving the decimal point one place left, then scale it: halve for 5%, double for 20%, add them for 15%.
- How do I calculate 15% quickly?
- Take 10%, then add half of it. For 86: 8.6 + 4.3 = 12.9.
- What is the swap trick for percentages?
- X% of Y equals Y% of X. So 8% of 50 is the same as 50% of 8, which is 4.
- Is 20% off then 10% off the same as 30% off?
- No. You pay 80% then 90% of that, which is 72% of the price, so the total discount is 28%.
- How do I calculate 1% of a number?
- Move the decimal point two places to the left. 1% of 4,800 is 48.
Calculators
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