Why Percentages Matter
Percentages are one of the most practical areas of mathematics. You encounter them daily — GST on your bill, discount on a sale, marks in an exam, interest on a loan, profit in a business.
Understanding how to calculate percentages quickly gives you a real advantage in everyday decisions.
Formula 1 — What is X% of Y?
This is the most common percentage calculation.
Formula: Result = (X / 100) x Y
Example: What is 20% of 500? = (20 / 100) x 500 = 0.20 x 500 = 100
Use cases:
- Calculating 18% GST on a Rs 1,000 bill = Rs 180
- Finding a 30% discount on a Rs 2,000 shirt = Rs 600 off
- Calculating 10% tip on a Rs 800 restaurant bill = Rs 80
Formula 2 — X is What Percent of Y?
Formula: Percentage = (X / Y) x 100
Example: 45 is what percent of 200? = (45 / 200) x 100 = 22.5%
Use cases:
- Finding your exam percentage: 85 marks out of 100 = 85%
- Calculating market share: 500 customers out of 5000 total = 10%
- Finding what percentage of your salary you save
Formula 3 — Percentage Change
Formula: % Change = ((New Value - Old Value) / Old Value) x 100
Example: Price increased from Rs 100 to Rs 120 = ((120 - 100) / 100) x 100 = 20% increase
Use cases:
- Tracking stock price changes
- Comparing this month's sales to last month
- Measuring weight loss or gain
Formula 4 — Add Percentage to a Number
Formula: Result = Number + (Number x Percentage / 100)
Example: Add 18% GST to Rs 1,000 = 1000 + (1000 x 18 / 100) = 1000 + 180 = Rs 1,180
Use cases:
- Adding GST to an invoice
- Calculating price after inflation
- Adding profit margin to cost price
Formula 5 — Remove Percentage from a Number
Formula: Original = Inclusive Amount x 100 / (100 + Percentage)
Example: Remove 18% GST from Rs 1,180 = 1180 x 100 / 118 = Rs 1,000
Use cases:
- Finding pre-GST price from GST-inclusive amount
- Finding original price before discount was applied
- Reverse calculating any percentage addition
Quick Percentage Reference Table
| Percentage | Fraction | Decimal | |-----------|---------|---------| | 10% | 1/10 | 0.10 | | 20% | 1/5 | 0.20 | | 25% | 1/4 | 0.25 | | 33.3% | 1/3 | 0.333 | | 50% | 1/2 | 0.50 | | 75% | 3/4 | 0.75 |
Free Percentage Calculator
Use our free Percentage Calculator to compute all 5 formulas instantly. No mental math required — just enter your values.
Conclusion
These 5 percentage formulas cover 99% of everyday percentage calculations. Practice them once and you will be able to calculate discounts, taxes, exam scores, and interest mentally. Or use our free Percentage Calculator for instant accurate results.