Enter values and select Calculate.
Calculated result
Formula and worked example
From radius, C = 2πr. From diameter, C = πd.
A circle with radius 5 has circumference 10π, approximately 31.4159. Entering diameter 10 produces the same result.
Calculations retain floating-point precision and use controlled display formatting. Decimal results involving π are approximations.
Radius or diameter
Radius measures from the center to the circle edge.
Diameter spans the circle through its center and equals twice the radius.
Choose the measurement you actually know before calculating.
Circumference versus area
Circumference is a linear distance around a circle.
Circle area is a squared measure inside it and appears only as a secondary reference.
Use Area Calculator when area is the primary job.
Zero and precision
Radius or diameter zero produces circumference zero.
Negative and nonfinite measurements are rejected.
Decimal values involving π are useful approximations and are not labeled exact.
Methodology and limits
Formulas are standard Euclidean geometry. Length conversions follow exact defined factors, including 1 inch = 0.0254 meter; area factors are squared and volume factors are cubed. This browser-local tool does not evaluate expressions or infer real-world material needs.