How to Calculate Percentages, Discounts, and Percentage Changes
September 6, 2026
Written by Mohammad Sohail
Percentage calculations come up constantly — discounts at the store, tips at restaurants, tax amounts, grade scores, business growth rates. Getting the math wrong costs real money. Here is how to do it correctly.
The Three Common Calculations: Finding X% of a number: Multiply the number by the percentage divided by 100. Example: 15% of 200 = 200 × 0.15 = 30. Finding what percent A is of B: Divide A by B, then multiply by 100. Example: 45 is what % of 180? = (45/180) × 100 = 25%. Percentage change: (New − Old) / Old × 100. Example: Sales went from 80 to 120 = (120−80)/80 × 100 = 50% increase.
Real-World Examples: A shirt costs $45 and is 30% off. Discount: $45 × 0.30 = $13.50. You pay $31.50. Your restaurant bill is $67 and you want to leave 18% tip. Tip: $67 × 0.18 = $12.06. Total: $79.06. Your website traffic went from 1,200 to 1,560 visitors. Growth: (1560−1200)/1200 × 100 = 30% increase.
What I Tested: I used the [Percentage Calculator](/calculator-tools/percentage-calculator) with several scenarios.
• What is 25% of 360? → 90. Correct.
• 45 is what percent of 180? → 25%. Correct.
• Percentage change from 200 to 260? → 30% increase. Correct.
• Percentage change from 500 to 420? → 16% decrease. Correct.
The calculator handles all three types and updates results instantly.
Step-by-Step: (1) Open the [Percentage Calculator](/calculator-tools/percentage-calculator). (2) Select the calculation type (find %, what %, or % change). (3) Enter your numbers. (4) Read the result instantly.
One Limitation: The calculator rounds results to two decimal places. For financial calculations requiring more precision, use a spreadsheet application like Excel or Google Sheets.
Common Mistake: Calculating percentage change in the wrong direction. Going from 100 to 150 is a 50% increase. Going from 150 to 100 is a 33.3% decrease — not 50%. The base (starting value) matters. Our calculator handles this correctly by using the starting value as the denominator.
Privacy: All calculations happen in your browser. No data is sent to any server.
Next Step: For more complex financial calculations, try the [Loan Calculator](/calculator-tools/loan-calculator) for EMI, interest, and payment schedules.