Percentages are everywhere — discounts, taxes, tips, test scores and statistics. Fortunately, every percentage question boils down to one simple relationship.
The basic formula
Percentage = (Part ÷ Whole) × 100
The 'part' is the amount you are comparing, and the 'whole' is the total it belongs to. Multiply by 100 to convert the decimal into a percent.
How to calculate a percentage of a number
To find X% of a number, multiply the number by X ÷ 100:
- 25% of 80 → 80 × 0.25 = 20
- 15% of 60 → 60 × 0.15 = 9
This is the exact math behind tips, discounts and sales tax.
How to find what percent one number is of another
Divide the part by the whole and multiply by 100:
- 45 out of 60 → 45 ÷ 60 × 100 = 75%
- 18 out of 40 → 18 ÷ 40 × 100 = 45%
This is how test scores, voter shares and survey results are computed.
Common percentage shortcuts
- 50% — divide by 2
- 25% — divide by 4
- 10% — divide by 10
- 5% — divide by 20
Working out 10% mentally then doubling gives you 20% almost instantly.
A worked example
A $120 jacket is marked 30% off. What is the sale price?
- Find the discount: 120 × 0.30 = $36
- Subtract it: 120 − 36 = $84
Put it into practice
Skip the mental math and run any percentage question — percent of, what percent, increase or decrease — on the free Percentage Calculator.
