5 Percentage Tricks You Can Do in Your Head

Percentages come up constantly — sale discounts, tips, grades, splitting a bill — and most of them are easier to work out in your head than people expect. Here are five shortcuts worth actually remembering.

1. To find 10%, just move the decimal point

10% of any number is that number divided by 10 — so 10% of 250 is simply 25. Once you have 10%, you can build almost any other percentage from it.

2. To find 20%, double your 10%

Since 20% is just two 10%s, once you know 10% of a number, doubling it gets you 20% instantly. 10% of 80 is 8, so 20% of 80 is 16.

3. To find 5%, halve your 10%

The reverse works too — half of 10% gives you 5%. If 10% of 60 is 6, then 5% of 60 is 3. Combine 10% and 5% and you can quickly build 15%, which is handy for tipping.

4. To find what percent one number is of another, simplify first

If you're trying to figure out what percent 45 is of 60, look for a simpler fraction first. 45 out of 60 simplifies to 3 out of 4, and 3/4 is 75% — often faster than doing long division.

5. For percentage increase or decrease, focus on the difference

If a price goes from 80 to 100, the increase is 20. Ask what percent 20 is of the original number, 80 — which is 25%. The "original" number is always what you divide by, not the new one.

When the numbers aren't round

These tricks work best with round numbers. For anything messier — like finding 17% of 342 — it's faster and more accurate to just use a proper percentage calculator instead of trying to force mental math.