Percentage formulas
"Per cent" means "per hundred", so 25% is 25 out of every 100, or 0.25. Every percentage problem is one of a few simple formulas:
| Question | Formula | Example |
|---|---|---|
| X% of Y | Y × X ÷ 100 | 15% of 2,400 = 360 |
| X is what % of Y | X ÷ Y × 100 | 45 of 60 = 75% |
| % change | (New − Old) ÷ Old × 100 | 50,000 → 56,500 = +13% |
| Price after discount | Price × (1 − D ÷ 100) | 1,999 at 30% off = 1,399.30 |
| Marks percentage | Obtained ÷ Maximum × 100 | 438 / 500 = 87.6% |
Common percentage mistakes
- Percent change isn't reversible. A 50% drop followed by a 50% rise does not get you back where you started: 100 → 50 → 75.
- Two discounts don't add up. "20% + 10% off" is 28% off in total, not 30%, because the second discount applies to the already-reduced price.
- Percent vs percentage points. An interest rate going from 8% to 10% is a rise of 2 percentage points, but a 25% increase in the rate.
Converting percentage to CGPA
Many Indian boards use the rule percentage = CGPA × 9.5 (for example, CGPA 9.2 ≈ 87.4%). Universities vary, so check your institution's official formula.
Calculating tax on a price? Use the GST calculator. Calculating investment growth? Try the compound interest calculator.
Frequently asked questions
How do I calculate a percentage of a number?
Multiply the number by the percentage and divide by 100. For example, 20% of 750 = 750 × 20 ÷ 100 = 150.
How do I calculate percentage increase?
Subtract the old value from the new value, divide by the old value and multiply by 100. From 80 to 100 is (100 − 80) ÷ 80 × 100 = 25%.
How do I calculate my marks percentage?
Divide your total marks by the maximum marks and multiply by 100. 438 out of 500 is 87.6%.
How do I work out the original price before a discount?
Divide the sale price by (1 − discount ÷ 100). A ₹700 item at 30% off had an original price of 700 ÷ 0.7 = ₹1,000.
