Math Calculators
Circle Calculator
Find a circle's area, circumference, and diameter.
Free account14-day Pro trialWorks in your browser
Sign in to use this tool
Create a free account to unlock every tool — plus a 14-day free trial of HQ Pro, no card required.
About the Circle Calculator
Enter a circle's radius and instantly get its area (πr²), circumference (2πr), and diameter (2r). It's a quick geometry helper for homework, design, and any project involving round shapes — no need to remember which formula uses which constant.
How to use the Circle Calculator
- 01Enter your radius.
- 02Read the result instantly — it recalculates as you type.
- 03Adjust the numbers to model different scenarios.
Frequently asked questions
What is the area of a circle?
Area = π × radius². A circle with radius 5 has an area of about 78.54 square units.
How do I find circumference?
Circumference = 2 × π × radius (or π × diameter). This tool shows both circumference and diameter.