Circle calculator.
Use this free circle calculator to get fast estimates, compare scenarios, and understand the numbers with clear step-by-step context.
Use the Circle Calculator.
Results update automatically as you type. No account is required.
Enter your values
About the Circle Calculator
A circle calculator finds every key property of a circle — radius, diameter, circumference, and area — from any one of them. Choose what you know, enter its value, and the rest are derived instantly using pi.
How it works
All properties trace back to the radius: diameter = 2r, circumference = 2πr, and area = πr². If you start from the area or circumference, the calculator first solves for the radius and then computes the others.
Reading the result
The headline result is the area, with radius, diameter, and circumference shown alongside so you can read off whichever you need.
Note: Calculations run instantly in your browser and use exact mathematical formulas. For very large numbers, results are shown to standard floating-point precision.
FAQ
What is pi?
Pi (π ≈ 3.14159) is the ratio of a circle’s circumference to its diameter. It is the same for every circle, which is why it appears in all these formulas.
Can I start from the area?
Yes. Select "Area" as your given value and the calculator works backward to the radius, then forward to diameter and circumference.
Diameter vs radius?
The diameter is the full width across the centre; the radius is half of it. Many formulas use the radius, so the calculator converts automatically.