Circle Calculator
Find radius, diameter, circumference, and area from any one value.
Radius
5
Diameter
10
Circumference
31.415927
Area
78.539816
Everything follows from r
All four circle measures interconvert through the radius: d = 2r, C = 2πr, A = πr². Given any one, the rest follow — from area, r = √(A/π); from circumference, r = C/2π. π ≈ 3.14159 is the one constant tying them together.
π in practice
π is the ratio of any circle's circumference to its diameter — the same for a coin and a planet. For estimation, C ≈ 3d is within 5%: a tree trunk you can just wrap arms around (~1.5 m) is about half a meter thick.
Frequently asked questions
- How does the Circle Calculator work?
- Find radius, diameter, circumference, and area from any one value.
- Is the Circle Calculator free to use?
- Yes. The Circle Calculator is completely free, runs entirely in your browser, and requires no account or sign-up. Your inputs never leave your device.
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