Free Circle Calculator Online
This free online circle calculator computes area, circumference, diameter, and radius from any single known value of a circle.
Circle Input
Area
Enter a value above
How the Circle Calculator Works
The circle calculator takes any one circle measurement and derives all others. Enter the radius, diameter, area, or circumference and the circle calculator instantly computes the remaining three values using standard geometry formulas.
From radius r: diameter = 2r, circumference = 2πr, area = πr². From diameter d: radius = d/2, circumference = πd, area = π(d/2)². The circle calculator handles both directions.
From area A: radius = √(A/π), diameter = 2√(A/π), circumference = 2π√(A/π). From circumference C: radius = C/(2π), diameter = C/π, area = C²/(4π). The circle calculator performs all conversions.
Circle Formulas Reference
Area
A = πr²
Circumference
C = 2πr = πd
Diameter
d = 2r
Radius from Area
r = √(A / π)
Tips for Using the Circle Calculator
Enter Any Value
The circle calculator derives all values from any single measurement. Just select the input type and enter the value you know.
Check the Visual
The circle calculator shows a proportional SVG diagram of the circle with labeled radius and diameter for quick visual verification.
Use Consistent Units
Area will be in squared units. If you enter radius in cm, area is cm², circumference is cm. The circle calculator uses whatever unit you provide.
Real-World Uses
Use the circle calculator for pipe cross-sections, pizza sizes, wheel measurements, pool covers, and any circular object.
Frequently Asked Questions
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