How to Calculate Percentage: 4 Methods With Examples
Learn how to calculate percentages four ways: percent of a number, what percent, percent change, and reverse percentage. Includes formulas and worked examples.
Percentage calculations come up constantly — in shopping discounts, tax calculations, grade curves, and financial analysis. Here are the four core methods you need to know, each with the formula and a worked example.
Method 1: Find X% of a Number
Formula: Result = Number × (Percentage ÷ 100)
Example: What is 15% of $240?
$240 × (15 ÷ 100) = $240 × 0.15 = $36
Use this for: calculating tips, sales tax, discounts, commission, and interest payments.
Method 2: What Percent is X of Y?
Formula: Percentage = (X ÷ Y) × 100
Example: 45 is what percent of 180?
(45 ÷ 180) × 100 = 0.25 × 100 = 25%
Use this for: finding your test score percentage, market share, or any ratio expressed as a percent.
Method 3: Percent Change
Formula: % Change = ((New − Old) ÷ Old) × 100
Example: A stock price went from $50 to $65. What is the percent increase?
((65 − 50) ÷ 50) × 100 = (15 ÷ 50) × 100 = 30% increase
A negative result means a decrease. Example: $50 to $40 = ((40−50)÷50)×100 = −20% (a 20% decrease).
Method 4: Reverse Percentage (Find the Original)
Formula: Original = Final ÷ (1 − discount rate)
Example: A shirt costs $68 after a 15% discount. What was the original price?
$68 ÷ (1 − 0.15) = $68 ÷ 0.85 = $80
This is the most frequently misused calculation. Many people subtract 15% from $68 and get the wrong answer ($57.80). The correct method divides by (1 − rate).
Quick Reference: Percentage Shortcuts
- 10% of any number: move decimal one place left (10% of 340 = 34)
- 1% of any number: move decimal two places left (1% of 340 = 3.40)
- 5% = half of 10% (5% of 340 = 17)
- 20% = divide by 5
- 25% = divide by 4
- 33.3% ≈ divide by 3
- 50% = divide by 2
Common Mistakes to Avoid
Mistake 1: Adding percentages directly. A 20% increase followed by a 20% decrease does not return to the original — it leaves you at 96% of the start.
Mistake 2: Confusing percent and percentage points. If a bank raises interest rates from 2% to 3%, it rose 1 percentage point but increased by 50 percent.
Mistake 3: Reversing a discount by subtracting. To undo a 20% discount, divide by 0.80, not subtract 20% from the discounted price.
Try our Percentage Calculator to do this calculation instantly — no formulas needed.