Percentage Calculator
Percent of, percent change, and percent increase.
What is X% of Y?
A is what % of B / % change
How this calculator works
Formula
part = (percent/100) × whole ; change = (b − a)/a × 100
Method
Three modes: percent-of, ratio-to-percent, and percent change between two values.
Example
20% of 150 = 30. Going from 80 to 120 is a 50% increase.
The complete guide to the Percentage Calculator
Percentages are the arithmetic of everyday life — tips, sales tax, discounts, and rate changes. This tool solves the three most common percentage problems instantly, and the explainer below shows the shortcuts you can do in your head.
What the calculator does
It answers three questions: what is X% of Y, A is what percent of B, and what is the percent change from A to B. Toggle inputs and results update in real time.
The three formulas
part = (percent/100) × whole. percent = (part / whole) × 100. change = ((new − old) / old) × 100. Every practical percentage problem is one of these three.
The swap trick
X% of Y equals Y% of X. Need 18% of 50? Do 50% of 18 = 9. Same answer, easier arithmetic. Use it constantly.
Mental math for tips
10% is the decimal shift. 15% is 10% plus half of 10%. 20% is 10% doubled. That covers most restaurant math without a phone.
Discount stacking is not additive
A 20% coupon on a 30% off item is not 50% off. Multiply the remaining percentages: 0.7 × 0.8 = 0.56 remaining, i.e., 44% off.
Percent change vs percent points
Interest rates going from 4% to 5% is a 1 percentage-point rise but a 25% relative increase. News reports mix these two constantly — always ask which.
Real-world use cases
Sales tax, tip calculation, sale prices, grade averages, growth reporting, body-fat calculations, macro splits, and every dashboard KPI. Percentages are the universal unit of comparison.
Frequently asked questions
How do I calculate percent decrease?
Same formula: ((new − old) / old) × 100. A negative result means a decrease.
How is percent different from percentile?
Percent is a fraction of a whole. Percentile is your rank in a distribution (95th percentile means you scored above 95% of others).