Circle Calculator
Know one measurement of a circle? Get the radius, diameter, circumference and area at once.
Reviewed by the OmniCalc teamMethod verified 2026-07-01
Known value
Result
78.54
Area 78.54- Radius
- 5
- Diameter
- 10
- Circumference
- 31.416
- Area
- 78.54
Show steps
- Given radius = 5, radius r = 5.
- Diameter = 2r = 10; circumference = 2πr = 31.416.
- Area = πr² = 78.54.
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How to use the circle calculator
- 1Choose which value you know — radius, diameter, circumference or area.
- 2Enter that value.
- 3Read the other three measurements, with the formulas under Show steps.
Worked example
A circle with radius 5 has diameter 10, circumference 2π·5 ≈ 31.42, and area π·5² ≈ 78.54 — all in the same units you entered.
Frequently asked questions
What are the circle formulas?
With radius r: diameter = 2r, circumference = 2πr, and area = πr². This tool inverts whichever value you know to recover r first, then computes the rest.
Can I start from the area or circumference?
Yes. Pick which value you know from the selector. From area, r = √(A/π); from circumference, r = C/(2π). Every other measure follows.
What value of π is used?
The full double-precision constant Math.PI, so results are accurate to about 12 significant figures before display rounding.