The base trick: 10%
10% of any number is calculated by moving the decimal point one place to the left. 10% of 250 is 25. From there, many others come easily:
- 20% = double the 10%.
- 5% = half the 10%.
- 15% = the 10% + half of it (5%).
- 1% = move the decimal point two places.
The number-swap trick
Mathematically, "X% of Y" is exactly the same as "Y% of X". This is very useful when one of the two numbers is "round". For example, calculating 4% of 25 is awkward in your head, but 25% of 4 is trivial (1). The result is the same: 1.
Calculating a quick tip
To leave a 10% tip on a restaurant bill, just move the decimal point. For 15%, add half of that already-calculated 10%.
The most common mistake: adding and subtracting percentages in a row
If a price goes up 20% and then down 20%, it doesn't return to the original price — it ends up lower than the original, because the second percentage is calculated on a base that's already different (larger) than the first.
For calculations that need precision
These tricks are great for quick estimates, but if you need an exact figure (for example, for an invoice or a report), better use the percentage calculator, which calculates to the cent and avoids mental rounding errors.