Guide
How to Calculate Percentage Increase and Decrease Correctly
Percentage calculations are used everywhere — from shopping discounts and salary raises to stock market changes and grade curves. This guide breaks down the formulas with clear examples so you can calculate percentage increase, decrease, and change confidently.
Last updated: April 9, 2026
The Three Most Common Percentage Calculations
Most real-world percentage questions fall into three categories: finding a percentage of a number (e.g., 'What is 20% of 150?'), finding what percentage one number is of another (e.g., '30 is what percent of 200?'), and calculating percentage change between two values (e.g., 'A price went from $80 to $100 — what's the percent increase?').
Each calculation uses a slightly different formula, but they are all simple once you understand the logic. Our Percentage Calculator at /calculators/percentage-calculator handles all three types instantly.
Formula 1: Finding X% of a Number
Formula: Result = (Percentage / 100) × Number
Example: What is 15% of 200? Answer: (15 / 100) × 200 = 0.15 × 200 = 30.
This is the most straightforward percentage calculation. You use it for tips (15% of $50 = $7.50), sales tax (8% of $120 = $9.60), and discounts (25% off $80 = $20 savings).
Formula 2: Finding What Percent X Is of Y
Formula: Percentage = (Part / Whole) × 100
Example: 45 is what percent of 180? Answer: (45 / 180) × 100 = 25%.
You use this when you know the part and the whole and want to find the ratio. Common use cases include test scores (you got 42 out of 50 — that's 84%), budget tracking (spent $300 of $2,000 budget — that's 15%), and completion rates (finished 7 of 10 tasks — that's 70%).
Formula 3: Percentage Change (Increase or Decrease)
Formula: Percentage Change = ((New Value − Old Value) / Old Value) × 100
If the result is positive, it's an increase. If negative, it's a decrease.
Example 1 (Increase): Price went from $80 to $100. Change = ((100 − 80) / 80) × 100 = 25% increase.
Example 2 (Decrease): Price went from $100 to $75. Change = ((75 − 100) / 100) × 100 = −25% decrease.
This is the formula used for salary raises, inflation rates, stock market changes, population growth, and any scenario where you're comparing an old value to a new one.
Common Mistakes to Avoid
- Using the wrong base number: For percentage change, always divide by the ORIGINAL (old) value, not the new value. Dividing by the new value gives a different result.
- Confusing percentage points with percentages: If an interest rate goes from 5% to 7%, that's a 2 percentage point increase but a 40% increase in the rate itself.
- Forgetting to convert: When using percentages in formulas, always divide by 100 first. 25% = 0.25, not 25.
- Stacking discounts incorrectly: 20% off plus 10% off is NOT 30% off. It is 20% off, then 10% off the reduced price. $100 → $80 → $72 (28% total, not 30%).
Real-World Examples
- Salary raise: Your salary goes from $50,000 to $55,000. Percentage increase = ((55,000 − 50,000) / 50,000) × 100 = 10%.
- Shopping discount: A $120 jacket is 30% off. You save: (30/100) × 120 = $36. You pay: $120 − $36 = $84.
- Restaurant tip: 18% tip on a $65 bill. Tip = (18/100) × 65 = $11.70. Total = $76.70.
- Grade calculation: You scored 86 out of 100. Percentage = (86/100) × 100 = 86%.
- Investment return: You bought stock at $40, now worth $52. Return = ((52 − 40) / 40) × 100 = 30%.
Use Our Free Calculator
You don't need to do this math by hand. Our Percentage Calculator at /calculators/percentage-calculator handles all three calculation types instantly. Just enter your numbers and get the result. It works on any device — desktop, tablet, or phone — with no signup required.
For discount-specific calculations with price breakdowns, try our Discount Calculator at /calculators/discount-calculator.
Frequently Asked Questions
- Q: How do I calculate percentage on a calculator? — Multiply the number by the percentage, then press the % button. Or divide the percentage by 100 and multiply manually. For example, 20% of 150: enter 150 × 20 ÷ 100 = 30.
- Q: What's the difference between percent and percentage? — 'Percent' means 'per hundred' and is used with a number (e.g., 50 percent). 'Percentage' refers to the concept in general (e.g., 'a large percentage of voters').
- Q: Can percentages be over 100%? — Yes. If something more than doubles, the percentage increase exceeds 100%. For example, a value going from 50 to 150 is a 200% increase.
- Q: How do I reverse a percentage? — To find the original number before a percentage was applied, divide by (1 + percentage/100) for increases or (1 − percentage/100) for decreases. Example: A price after 25% increase is $100. Original = 100 / 1.25 = $80.
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