Circumference Calculator
Circumference calculator: find the circumference, diameter and area of a circle from its radius or its diameter, with the formulas shown.
Editorial review: Farrow (pseudonym; identity and professional credentials are not disclosed). This page is for educational purposes, not professional advice.
Your inputs
Results update as you type. Nothing you enter leaves your device.
Results
- Diameter
- 10
- Area
- 78.5398
Example with the starting values
These results use the calculator's initial inputs. Change the values above to calculate your own estimate.
Inputs
- I know the
- Radius
- Length
- 5
Results
- Circumference
- 31.4159
- Diameter
- 10
- Area
- 78.5398
How does the Circumference Calculator work?
Circumference = 2πr = πd, where r is the radius and d the diameter. A circle with a 5 inch radius has a 10 inch diameter and a circumference of 2 × 3.14159 × 5 = 31.416 inches.
Area = πr². The same circle has an area of 78.54 square inches. Enter a diameter instead by switching the input type.
Frequently asked questions
What is the circumference formula?
C = 2πr or C = πd. For a 10-inch diameter, C = 3.14159 × 10 = 31.416 inches.
How do I find the circumference from the area?
Radius = √(area / π), then C = 2πr. An area of 78.54 gives r = 5 and C = 31.416.
What is pi?
Pi (π) is the ratio of circumference to diameter for any circle, about 3.14159, and it never ends or repeats.
How do I find the diameter from the circumference?
Divide the circumference by π. A circumference of 31.416 gives a diameter of 10.