How to work out a percentage in your head
You do not need a calculator for most everyday percentages. Almost all of them can be built from one easy step: finding 10%. Once you can do that, 5%, 15%, 20% and 25% are one small step away.
Step one: find 10%
To find 10% of any number, move the decimal point one place to the left. 10% of 64 is 6.4. 10% of 85 is 8.5. 10% of 240 is 24. That is the whole trick, because 10% means one part in ten.
Step two: build the rest from it
| Percentage | How to build it | Example on $80 |
|---|---|---|
| 5% | Half of 10% | $8.00 halved is $4.00 |
| 10% | Move the decimal point | $8.00 |
| 15% | 10% plus 5% | $8.00 + $4.00 = $12.00 |
| 20% | Double 10% | $16.00 |
| 25% | A quarter of the number | $80 ÷ 4 = $20.00 |
Worked examples
Three quick sums
15% of $64: 10% is $6.40, 5% is $3.20, so 15% is $9.60.
20% of $85: 10% is $8.50, doubled is $17.00.
5% of $240: 10% is $24, halved is $12.
Where this helps
- A tip: work out 10% of the bill, then add half of it again for 15%, or double it for 20%.
- A sale: 20% off a $85 jacket takes off $17.00, so you pay $68.00.
- A quick check: if a shop says 25% off, you pay three quarters of the price.
When the number is awkward
Round first, then adjust. For 15% of $47.80, think of $48: 10% is $4.80, 5% is $2.40, so about $7.20. The exact answer is $7.17, close enough to decide a tip. If the exact amount matters, such as on a receipt or a price check, use a calculator.
A common slip
Taking 20% off and then adding 20% back does not return you to the start. $100 less 20% is $80, and 20% more than $80 is $96, not $100. Each percentage works on the number in front of it.
Tip Calculator: Check a tip in your head, then confirm the exact amount and the split per person.
Discount Calculator: Enter a price and a discount to see the sale price and how much you save.