Toolstia / Circle calculator
Circle calculator
Know one measurement of a circle and want the rest? Enter the radius, diameter, circumference or area and the other three are worked out instantly.
How it works
- Choose what you know.
- Enter the value.
- Read the radius, diameter, circumference and area.
Formulas
Diameter = 2r. Circumference = 2πr = πd. Area = πr².
Working backwards
From the circumference, r = C ÷ 2π. From the area, r = √(A ÷ π).
About π
π is about 3.14159265. The tool uses the full precision available in your browser.
Units
Area is in squared units of whatever unit the length was in.
Questions
How do I find the radius from the area?
Choose Area, enter it, and read the radius.
What is the ratio of circumference to diameter?
It is always π, for every circle.
Is anything uploaded?
No. Your input stays in your browser and is never sent anywhere.