Percentage Calculator – Complete Guide & Free Online Tool
A percentage calculator solves common percentage problems: what is X% of Y, X is what percent of Y, increase or decrease by a percentage, and percentage difference — all with clear formulas. Free online tool included.
Published 2026-09-13
What is Percentage Calculator?
A percentage calculator solves common percentage problems: what is X% of Y, X is what percent of Y, increase or decrease by a percentage, and percentage difference — all with clear formulas.
What is a percentage?
A percentage is a fraction of 100. "X% of Y" means (X/100) × Y. Percentages are used to express proportions, changes and comparisons.
Key formulas
Percent of a number: (A/100) × B. What percent A is of B: (A/B) × 100. Increase A by B%: A × (1 + B/100). Decrease A by B%: A × (1 − B/100). Percentage difference: |A−B| / ((A+B)/2) × 100.
Common mistakes
A common error is adding a percentage instead of multiplying. For example, increasing 100 by 10% gives 110 (not 110% of 100 confused with 100 + 10). Percentage difference uses the average as the base, not one of the values.
Key Features
• Five common percentage modes • Clear formulas • Invalid input validation • Instant results
Examples
• 15% of 200 = 30. • 30 is 15% of 200. • 100 increased by 10% = 110. • Percentage difference between 100 and 80 is 22.2%.
Common Use Cases
• Discounts • Grades • Statistics • Financial analysis
Tips & Best Practices
• For a 20% discount on a $50 item • the price is 50 × (1 − 0.20) = $40. • Understand whether a percentage is based on the original or the new value.
Limitations
• Results are exact within standard floating-point precision.
External Resources
Wikipedia: Percentage