The Complete Guide to Percentage Calculations
Five essential percentage formulas explained with real-world examples from shopping, finance, and data analysis.
1. The Five Core Formulas
| Mode | Formula | Example |
|---|---|---|
| Percentage Of | Result = (X ÷ 100) × Y | 20% of 150 = 30 |
| Percentage Change | Change = ((B − A) ÷ A) × 100 | From 80 → 100 = +25% |
| What Percent? | Result = (X ÷ Y) × 100 | 25 is 12.5% of 200 |
| Add Percentage | Result = Y × (1 + X ÷ 100) | $50 + 20% = $60 |
| Subtract Percentage | Result = Y × (1 − X ÷ 100) | $100 − 15% = $85 |
2. Why Percentage Change is Asymmetric
Going from 100 to 50 is a 50% decrease, but going from 50 to 100 is a 100% increase. Percentages always express the change relative to the starting value, not the ending value. This asymmetry is critical to understand when comparing investment returns, price changes, or grade improvements.
3. Add vs. Subtract Percentage
- Add Percentage is used for: sales tax, VAT, markup, interest added to principal
- Subtract Percentage is used for: discounts, markdowns, depreciation, salary deductions
- Note: a 20% increase followed by a 20% decrease does not return to the original — it results in a 4% net decrease
4. Privacy
🔒 Browser-Only Computation
All percentage calculations run entirely in your browser. No values are uploaded to any server or logged by analytics.