Percentage Calculator, Find %, Increase, Decrease & Difference

Finding a Percentage of a Number

Formula: Result = Number × Percentage / 100

• 20% of 150 = 150 × 20 / 100 = 30
• 15% tip on $80 = $80 × 15 / 100 = $12
• 8.5% tax on $50 = $50 × 8.5 / 100 = $4.25

Finding What Percent X is of Y

Formula: Percentage = (Part / Whole) × 100

• 18 out of 90 = (18 / 90) × 100 = 20%
• Test score: 42 out of 50 = 84%

Percentage Change

Formula: Change = ((New - Old) / Old) × 100

• $50 to $60 = ((60 - 50) / 50) × 100 = +20%
• $100 to $75 = ((75 - 100) / 100) × 100 = -25%

Mental Math Shortcuts

• 10%: Move the decimal point one place left
• 5%: Find 10% and divide by 2
• 25%: Divide by 4
• 50%: Divide by 2
• 1%: Move the decimal point two places left

When to Use This Calculator

• Discounts and sales: Find how much a 30% discount saves on a $129 item, instantly.
• Year-over-year growth: Measure revenue, traffic, or salary changes between two time periods.
• Grade calculation: Find what percentage of points you earned on a test or assignment.

Real-World Examples

Example 1, Sale price: Item costs $299, on sale at 35% off. Discount = $299 × 35 / 100 = $104.65. Sale price = $299 − $104.65 = $194.35.

Example 2, Investment return: Portfolio grew from $12,500 to $15,750. Change = ((15,750 − 12,500) / 12,500) × 100 = +26% return.

Limitations & Assumptions

• Percentage change requires the original value to be non-zero, division by zero is undefined.
• Percentages above 100% are valid (e.g., 150% increase means value 2.5x the original).
• Percentage points and percentages are different: going from 20% to 25% is a 5 percentage point increase but a 25% relative increase.

Data Sources

Formulas follow standard arithmetic definitions used by the US Bureau of Labor Statistics for CPI calculations and the SEC for investment return reporting. No external data source required.