Circle Calculator
Diameter, circumference, area and more from one radius.
Frequently Asked Questions
What is the relationship between radius and diameter?
The diameter is twice the radius and passes through the centre from one side of the circle to the other. Every circle formula can be written with either, but radius is the standard input.
Why does pi appear in every circle formula?
Pi is the constant ratio of circumference to diameter for every circle, about 3.14159. It appears in area (π r²) and sphere formulas because they are all built from that same circular geometry.
How do I get circumference from area?
First recover the radius: r = √(A / π). Then circumference is 2π r. For an area of 154, the radius is 7 and the circumference is about 43.98.
Is the Circle Calculator free, and is my data private?
Yes. It is completely free and every calculation runs locally in your browser, so the numbers and dates you enter are never uploaded, stored, or shared with anyone.
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