How to Use the Percentage Calculator
Pick the mode that matches how you ask the question, then fill the blanks. Results update as you type.
Choose a mode
Select percent of a number, “X is what % of Y,” percent change, increase/decrease by a percent, or “X is Y% of what.”
Enter the numbers
Type into each blank in the sentence. Negatives are allowed where the math is valid; leave a field empty to clear the result.
Read the result and step
The large answer appears on the right with a one-line formula filled with your values — useful for homework checks and receipts alike.
Percentage Formulas
Each mode uses a standard percent formula. Here is the math with one fixed example apiece.
What is X% of Y?
(X ÷ 100) × Y. Example: 15% of 200 → (15 ÷ 100) × 200 = 30.
X is what % of Y?
(X ÷ Y) × 100. Example: 40 is what % of 160 → (40 ÷ 160) × 100 = 25%.
Percent change from X to Y
((Y − X) ÷ |X|) × 100. Example: from 80 to 100 → ((100 − 80) ÷ 80) × 100 = 25% increase. Absolute value in the denominator keeps negative starts well-defined.
Increase or decrease X by Y%
X × (1 ± Y/100). Example: increase 80 by 25% → 80 × 1.25 = 100. Decrease 50 by 10% → 50 × 0.9 = 45.
X is Y% of what number?
X ÷ (Y ÷ 100). Example: 20 is 25% of what → 20 ÷ 0.25 = 80.
Common Uses
A percent calculator shows up anywhere prices, scores, or rates are compared.
Discounts and sale prices
Take 20% off a price or check what a coupon leaves you paying.
Sales tax and tips
Add tax to a subtotal or figure a tip as a percent of the bill.
Grades and test scores
Convert points earned into a percent of the total possible.
Price changes and raises
Measure percentage increase between old and new prices or salaries.
Interest and growth rates
Express growth as a percent of a starting balance when you compare two snapshots (simple percent change, not compound schedules).
Frequently Asked Questions
Percent of a number, percent change, and finding the whole.
How do I calculate a percentage of a number?
Use the “What is X% of Y?” mode. Enter the percent and the number — for example, what is 15% of 200. The calculator multiplies the percent by the number over 100, so 15% × 200 = 30, and shows that worked step under the result.
How do I find what percent one number is of another?
Choose “X is what % of Y?”, enter the part and the whole, and read the percent. The formula is (X ÷ Y) × 100. If the whole is zero, division is impossible and you will see a short message instead of a result.
How do I calculate percentage increase or decrease?
Use “% change from X to Y” for the relative change between two values, or “Increase / decrease X by Y%” when you want to apply a percent to a starting amount (such as a discount or raise). Percent change uses ((Y − X) ÷ |X|) × 100 and labels the result as an increase or decrease.
What is the difference between percentage change and percentage points?
Percentage change is relative to a starting value — moving from 20% to 30% is a 50% increase because 10 is half of 20. Percentage points measure the plain difference between two percents — that same move is 10 percentage points. This calculator reports percentage change, not percentage points.
How do I add or subtract a percentage from a number?
Open “Increase / decrease X by Y%”, pick Increase or Decrease, then enter the number and the percent. That covers markups, sale discounts, tax add-ons, and “subtract 10% from 50.” The worked step shows X × (1 ± Y/100).
How do I find the original number if I know a percentage of it?
Use “X is Y% of what number?” when you know a part and the percent it represents — for example, 20 is 25% of what number. The calculator divides the part by (percent ÷ 100) to recover the whole.
Why is percent change undefined when the starting value is zero?
Percent change divides by the starting value (using its absolute value so negative starts stay well-defined). If that start is zero, there is no baseline to compare against, so the ratio is undefined. Enter a non-zero starting value, or use another mode if you are not measuring change from a baseline.
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