Percentage Calculator
Four Calculation Modes
Understanding Percentages
"Percent" literally means "per hundred" — a percentage is just a fraction with a denominator of 100. Once you can move fluently between the four question types below, percentage math becomes second nature.
Sales tax, tips, and discounts all use this pattern: convert the percent to a decimal by dividing by 100, then multiply by the total.
Whether it's a stock price, a salary, or a population, percent change tells you the relative size of the move — always measured against the original starting value, not the new one.
The Four Formulas
(X ÷ 100) × Y
(X ÷ Y) × 100
((Y − X) ÷ X) × 100
X × (1 ± Y/100)
Key Takeaways
- Percent change is always relative to the starting value. A 50% drop followed by a 50% rise does not return you to the original number.
- Zero as a divisor is always undefined. The "is what percent" and "percent change" modes both guard against a zero denominator.
- "Plus/minus %" is the fastest way to price a markup or discount without doing the percent calculation and addition as two separate steps.
Frequently Asked Questions
- Choose the tab that matches the question you're trying to answer.
- Fill in the two numbers requested for that mode.
- Click "Calculate" to see the result.
Use the "X% of Y" tab. The formula is (X ÷ 100) × Y. For example, 20% of 50 is (20 ÷ 100) × 50 = 10.
Use the "X is what % of Y" tab. The formula is (X ÷ Y) × 100. For example, 10 is what percent of 50? (10 ÷ 50) × 100 = 20%.
Use the "% Change" tab. The formula is ((New − Old) ÷ Old) × 100. A positive result is a percent increase; a negative result is a percent decrease. Going from 50 to 75 is a 50% increase, while going from 100 to 50 is a 50% decrease.
This mode answers questions like "what is 200 plus 10%?" (a price increase or markup) or "what is 200 minus 10%?" (a discount). The formulas are X × (1 + Y/100) for plus and X × (1 − Y/100) for minus.
Both the "is what percent" and "percent change" calculations divide by that value. Dividing by zero is mathematically undefined, so the calculator blocks that input with a clear validation message instead of showing an incorrect result like Infinity.