How to Calculate Percentages
A percentage is a fraction out of 100. 15% means 15 out of every 100, or 0.15. Once you remember that, most percentage problems become simple multiplication or division.
Percent of a number
To find a percentage of a number, multiply the number by the percentage divided by 100.
Example: What is 15% of 80? Type 80 × 15 % =. The answer is 12.
Add a percentage (tax, tip or markup)
To add a percentage to an amount, add that percentage of the amount to itself.
Example: A $80 bill plus a 15% tip. Type 80 + 15 % =. The answer is 92. The % key turns 15 into 15% of 80 (which is 12) before adding.
Subtract a percentage (discount)
Example: A $60 jacket is 25% off. Type 60 − 25 % =. The sale price is 45, so you save 15.
What percent is one number of another?
Divide the part by the whole and multiply by 100.
Example: You answered 18 of 24 questions correctly. Type 18 ÷ 24 = to get 0.75, which is 75%.
Percent change
Percent change compares a new value with an old one: (new − old) ÷ old × 100.
Example: A price rises from 80 to 92. The difference is 12, and 12 ÷ 80 = 0.15, so the price rose by 15%.
| Question | Formula | Example |
|---|---|---|
| X% of a number | number × X ÷ 100 | 15% of 80 = 12 |
| Add X% | number × (1 + X ÷ 100) | 80 + 15% = 92 |
| Subtract X% | number × (1 − X ÷ 100) | 60 − 25% = 45 |
| Percent change | (new − old) ÷ old × 100 | 80 → 92 = +15% |
Common mistakes
- Reversing a change. A 20% increase followed by a 20% decrease does not return to the start. 100 becomes 120, then 96.
- Stacking discounts. Two 10% discounts are not a 20% discount. 100 becomes 90, then 81, which is 19% off.
- Using the wrong base. Percent change is always measured against the old value.
Tip: open two windows, one for the original price and one for the discounted price, and compare them at a glance.