What a percentage means
“Percent” means “per hundred.” 25% is 25 out of 100, which is the same as 0.25 or ¼. To use a percentage in math, divide it by 100.
1. Find a percentage of a number
Example: a 20% tip on $45 is 45 × 20 ÷ 100 = $9.
2. Find what percent one number is of another
Example: 42 correct answers out of 50 is 42 ÷ 50 × 100 = 84%.
3. Percent increase or decrease
Example: rent goes from $1,500 to $1,620. (1,620 − 1,500) ÷ 1,500 × 100 = 8% increase. A negative result is a decrease.
4. Add or subtract a percentage
Multiply by (1 + percent ÷ 100) to add, or (1 − percent ÷ 100) to subtract.
- A 4% raise on $52,000: 52,000 × 1.04 = $54,080.
- 30% off $80: 80 × 0.70 = $56.
5. Reverse percentages (find the original)
Example: a jacket costs $63 after 30% off. 63 ÷ 0.70 = $90 original price. Do not just add 30% back — $63 × 1.30 = $81.90, which is wrong.
Mental-math tricks
| To find | Do this | Example (of $60) |
|---|---|---|
| 10% | Move the decimal left one place | $6 |
| 5% | Half of 10% | $3 |
| 1% | Move the decimal left two places | $0.60 |
| 15% | 10% + 5% | $9 |
| 20% | 10% × 2 | $12 |
| 25% | Divide by 4 | $15 |
| 50% | Divide by 2 | $30 |
Percentages in Excel or Google Sheets
| Task | Formula |
|---|---|
| X% of a number | =A2*B2 (with B2 formatted as %) |
| Part as a percent | =A2/B2 then format as % |
| Percent change | =(B2-A2)/A2 |
| Add a percent | =A2*(1+B2) |
Skip the formulas and use the percentage calculator, which handles all four questions at once.
Questions
How do I calculate 15% of a number?
Multiply by 0.15. Or find 10% (move the decimal) and add half of it. 15% of $80 is $8 + $4 = $12.
How do I calculate my grade percentage?
Divide points earned by points possible and multiply by 100. 88 out of 110 is 80%.
What is the formula for percent increase?
(New value − old value) ÷ old value × 100.
How do I find the original price before a discount?
Divide the sale price by (1 − discount as a decimal). $63 after 30% off: 63 ÷ 0.70 = $90.