Circumference Calculator
Calculate circumference from radius or diameter.
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
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Request a ToolHow to Use the Circumference Calculator
This calculator finds the circumference (perimeter) of a circle from the radius or diameter.
- Choose your input type. Select whether you know the radius or the diameter.
- Enter the value. Type the measurement into the input field.
- Read the result. The circumference is shown instantly, along with the formula used and the corresponding radius and diameter values.
The formula for circumference from radius is C = 2 * pi * r. From diameter, it is C = pi * d. Both formulas give the same result since the diameter is twice the radius. Results update as you type.
About Circumference
The circumference is the distance around the outside of a circle. It is the circle's equivalent of a polygon's perimeter. The ratio of a circle's circumference to its diameter is the mathematical constant pi (approximately 3.14159). This relationship holds for every circle regardless of size. Circumference calculations are essential in engineering (wheel rotation, pipe sizing, belt lengths), construction (curved structures, circular foundations), and everyday tasks like measuring rings, circular tracks, and cylindrical containers.
Frequently Asked Questions
What is the circumference formula?
The circumference formula is C = 2 * pi * r (using radius) or C = pi * d (using diameter). For a circle with radius 5, the circumference is 2 * 3.14159 * 5 = 31.4159 units.
How do I find the radius from the circumference?
Divide the circumference by 2 * pi: r = C / (2 * pi). For example, if the circumference is 31.4159, the radius is 31.4159 / 6.2832 = 5. Use the Circle Calculator for this reverse calculation.
Is circumference the same as perimeter?
Yes, circumference is the specific term for the perimeter of a circle. Both refer to the total distance around the outside of the shape. "Perimeter" is the general term used for any shape, while "circumference" is used exclusively for circles.