Percentage Calculator -- Formulas and Examples
Why percentage math trips people up
Percentages show up everywhere -- discounts, tax rates, test scores, analytics dashboards, tip calculations. The math itself is simple division and multiplication, but there are three distinct types of percentage questions and mixing them up leads to wrong answers.
"What is 15% of 200?" is different from "30 is what percent of 200?" which is different from "What is the percentage change from 200 to 250?" Each question uses a different formula, and under time pressure it is easy to divide when you should multiply or forget to subtract before dividing.
A percentage calculator handles all three cases. You pick the question type, enter your numbers, and get the answer with the formula shown so you can verify it makes sense.
How to use the calculator
- Open the Percentage Calculator.
- Choose which type of calculation you need.
- Enter your numbers in the input fields.
- Read the result instantly -- no submit button needed.
The three types of percentage calculations
Type 1: What is X% of Y?
Formula: result = (X / 100) * Y
Example: What is 15% of 200? (15 / 100) * 200 = 30. Use this for discounts, tips, tax amounts, and "take a percentage of a total" questions.
Type 2: X is what percent of Y?
Formula: result = (X / Y) * 100
Example: 30 is what percent of 200? (30 / 200) * 100 = 15%. Use this for test scores (42 out of 50), conversion rates (12 sales from 400 visitors), and "what fraction as a percentage" questions.
Type 3: Percentage change from X to Y
Formula: result = ((Y - X) / X) * 100
Example: Change from 200 to 250? ((250 - 200) / 200) * 100 = 25%. Use this for price changes, growth rates, and before/after comparisons. A negative result means a decrease.
Percentage change vs percentage difference
These are not the same thing, and confusing them is a common mistake:
- Percentage change measures how much a value increased or decreased relative to the starting value. It has a direction -- from 100 to 150 is +50%, but from 150 to 100 is -33.3%.
- Percentage difference compares two values without a defined starting point. It uses the average of both values as the denominator:
|X - Y| / ((X + Y) / 2) * 100. The difference between 100 and 150 is 40% regardless of order.
Use percentage change when you have a clear "before" and "after." Use percentage difference when comparing two independent values, like the price of the same item at two stores.
Quick reference
- 10% of a number -- move the decimal one place left (10% of 450 = 45)
- 25% of a number -- divide by 4 (25% of 360 = 90)
- 33% of a number -- divide by 3 (33% of 270 = 90)
- 50% of a number -- divide by 2
- To reverse a percentage: if an item is $80 after a 20% discount, the original was
80 / 0.8 = $100
Built with vanilla HTML/JS. No frameworks, no backend, loads instantly.