Circumference from radius, diameter or area — with the full formula working.
Enter any one measurement and the rest of the circle is filled in, with the formula working shown.
Circumference
How to use this tool
Pick what you know: radius, diameter, area or circumference.
Type the value and pick its unit (cm, m, mm, inches or feet).
The circumference, radius, diameter and area all appear, with the formula working.
Change the unit to see the same circle measured in a different unit — the numbers convert with it.
Formula & worked example
A circle with radius 5 cm has diameter 10 cm, circumference 2π × 5 = 31.4159 cm, and area π × 5² = 78.5398 cm². Enter the radius 5 and all four values are filled in, with the working shown.
Limitations — read this before you rely on it
This is a perfect mathematical circle: real-world round objects are rarely perfectly circular, so measurements taken with a tape may differ slightly. Results use π to full precision and are rounded to 4 decimal places for display. Area is given in square units of the unit you chose.
Frequently asked questions
Use C = 2πr, where r is the radius. For radius 5 cm that is 2 × π × 5 = 31.4159 cm.
Use C = πd. If the diameter is 10 cm, the circumference is π × 10 = 31.4159 cm.
First find the radius: r = √(A ÷ π). Then use C = 2πr. For area 78.5398 cm², r = 5 cm and C = 31.4159 cm.
Yes. Halve the diameter to get the radius, then use C = 2πr — or use C = πd directly.
π (pi) is the constant ratio of a circle's circumference to its diameter, about 3.14159. That is why C = πd always works, for any circle.