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Circumference calculator for a circle

Find a circle’s circumference from its radius. The calculator applies C = 2πr and shows the result with a formula and example, without an account.

Units and conversions

Results update as you type.

Result

From a circle to a line

Unrolling the boundary gives a segment about 3.14 diameters long. Change the radius: circumference grows in the same proportion.

How do you calculate circumference?

Circumference measures a circle’s boundary rather than the space inside it. Double the radius and multiply by π; when you know the diameter, the doubling is already done.

C = 2 × π × r
r = d 2

How to use the calculator

Enter measurements with a decimal point or comma: results update immediately. Input units apply to every length; changing them converts the measurements. Changing result units leaves the shape unchanged. The control below the drawing reveals or hides its visual explanation. Copy results includes valid measurements and the result. Reset and Try this example restore the initial values and centimetre units.

Choose radius or diameter: switching preserves the shape’s dimensions. Drawing handles always change the radius and also update the diameter when selected.

The first formula shows the relationship; the second substitutes measurements converted to the result unit. The ≈ sign indicates rounding, including in the substituted values.

Edging a circular disc

A disc with radius 3 cm has diameter 6 cm. Covering its exact boundary takes about 18.85 cm of ribbon; add any overlap separately.

From measurements to the result

Input
r = 3 cm
Result
≈18.85 cm

Calculation steps

  1. Double the radius to obtain the diameter.

    d = 2 × 3 = 6

  2. Complete the calculation using the tool’s formula.

    C = 6 × π

Units and interpretation

A boundary is a length, measured in cm, m or another linear unit. One metre equals 100 centimetres.

Enter radius or diameter directly. Ribbon thickness is not included.

Precision and numerical limits

Lengths range from 0 to 10¹² in the selected unit; triangle sides must be strictly positive. Results use two decimal places, with scientific notation for very small or large values. Calculations use browser precision: unrepresentable positive values produce an error. A rejected unit change keeps the previous data. Calculation takes place in your browser.

Maximum diameter is 10¹² in the input unit. A rejected mode change keeps the previous data.

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Trapezoid area

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