Percentage Calculator

Calculate percentages easily with ThisDevTool's free online percentage calculator.

Last reviewed: April 2026
Reviewed by , software engineer · last updated April 2026

New to this tool? Click here for instructions

Select a mode and enter values to calculate.

How to Use the Percentage Calculator

The Percentage Calculator is a versatile tool that helps you perform various percentage-related calculations. To use it, follow these steps:

1. Select the mode that matches the question you are trying to answer. The tool supports five common cases: finding X% of Y, finding what percent X is of Y, calculating percentage change, applying an increase or decrease by percent, and converting a fraction to percent.

2. Enter the numbers in the provided fields.

3. Read the result label and use the bar chart as a quick visual check.

4. If you need a clean sentence for notes or chat, use the 'Copy Result' button.

  • Select the appropriate mode
  • Enter the required numbers
  • Read the result and bar chart
  • Copy the result for notes or chat

Where It Helps in Real Work

This tool is useful anywhere ratios need to be explained quickly. It can be used in various scenarios such as pricing changes, discounts, test scores, analytics lift, conversion rates, tax calculations, recipe scaling, or performance deltas between two measurements. It is also helpful when you are checking another system's math. For example, if a dashboard says revenue is up 12.4% or a spreadsheet says a discount should land at a certain number, you can verify the formula here in a few seconds.

How the Math Works

The formulas used in the Percentage Calculator are straightforward and all run in the browser:

1. X% of Y: (X / 100) * Y

2. What percent X is of Y: (X / Y) * 100

3. Percentage change: ((New Value - Old Value) / Old Value) * 100

4. Increase/Decrease by %: New Value = Original + (Original * (Percent / 100))

5. Fraction to %: (Numerator / Denominator) * 100

Tips and Limitations

Division by zero is invalid, so the tool will refuse results when the denominator or original value is zero in the relevant modes.

Large increases can exceed 100%, which is normal. A jump from 20 to 60 is a 200% increase.

Negative values are allowed, but interpret percentage change carefully when your starting value is below zero.

The calculation happens locally in the page. There is no server call behind the result.

Frequently Asked Questions

To calculate X% of Y, multiply Y by X and divide by 100. For example, 25% of 200 = (25 * 200) / 100 = 50.
Percentage change = ((New Value - Old Value) / Old Value) * 100. A positive result means an increase; a negative result means a decrease.
To find what percentage X is of Y, divide X by Y and multiply by 100. For example, 30 is what percent of 120? (30 / 120) * 100 = 25%.
To increase a number by a percentage: New Value = Original + (Original * (Percent / 100)). For example, increasing 400 by 25% gives 400 + (400 * (25 / 100)) = 500.
To convert a fraction to a percentage, divide the numerator by the denominator and multiply by 100. For example, 3/8 = (3 / 8) * 100 = 37.5%.

Quick reference

Percentage Calculator Quick Reference
Percentage Base Number Result Comparison
10% 200 20 20 is 10% of 200
25% 80 20 20 is 25% of 80
50% 100 50 50 is half of 100
1/3 90 30 30 is 33.33% of 90
75% 120 90 90 is 75% of 120
15% 50 7.5 7.5 is 15% of 50

Example walk-through

Worked example: step-by-step

Step 1. Navigate to the Percentage Calculator webpage and locate the input fields for "Percentage" and "Number".

Step 2. Enter the values: for example, input "20" in the Percentage field and "150" in the Number field.

Step 3. Click the "Calculate" button to trigger the computation logic.

Step 4. The webpage will display the result: "20% of 150 is 30".



Percentage: 20
Number: 150



Result: 20% of 150 is 30

Step 5. Verify the calculation by cross-checking with manual computation: 20/100 * 150 = 30.