How to use the circumference calculator
- 1 Choose what you know: radius, diameter, circumference or area.
- 2 Enter the value.
- 3 Choose the unit: inches, feet, centimeters or meters.
- 4 Read the circumference and the other measurements of the circle.
Worked example
A circle with a radius of 5 inches. The circumference is C = 2πr = 2 × π × 5 = 31.4159 inches (31.4 with π ≈ 3.14). The diameter is 10 inches and the area is πr² = 78.5398 square inches. A half circle arc is 15.708 inches.
Measuring circles accurately
- Measure the diameter straight across through the center.
- For pipes and trees, wrap a tape around and enter the circumference.
- Use exact π unless your class asks for 3.14.
- Keep units the same for every measurement.
Find a tank's capacity with the Cylinder Volume Calculator, the area of a round patio with the Square Footage Calculator, and convert lengths with the Length Converter.
How it works: Whatever you enter is turned into the radius first: the diameter is halved, a circumference is divided by 2π and an area is divided by π before taking the square root. All other values are then worked out from the radius.
Features
Any starting value
Radius, diameter, circumference or area: the rest are worked out.
Shows the formula
C = 2πr with your numbers filled in.
π or 3.14
Exact answer and the textbook 3.14 version.
When to use it
Homework
Check circle problems and show the working. Find hypotenuse lengths with the Pythagorean Theorem Calculator.
Measuring round objects
Wrap a tape around a pipe or tree, enter the circumference and get the diameter.
Cylinders and tanks
Use the radius to find a tank's capacity with the Cylinder Volume Calculator.
Floors and patios
Find the area of a round patio with the Square Footage Calculator.
Related Math tasks
Frequently asked questions
What is the formula for circumference?
C = 2πr, where r is the radius, or C = πd, where d is the diameter. π is about 3.14159.
How do I find the circumference from the diameter?
Multiply the diameter by π. A circle 10 inches across has a circumference of about 31.42 inches.
How do I find the radius from the circumference?
Divide the circumference by 2π. A circumference of 31.42 inches gives a radius of 5 inches.
How do I find the circumference from the area?
First find the radius: r = √(A ÷ π). Then C = 2πr. An area of 78.54 square inches gives a radius of 5 and a circumference of 31.42 inches.
What is the difference between circumference and perimeter?
Circumference is the perimeter of a circle: the distance all the way around it. Perimeter is the general word for any shape.
Why does my textbook answer differ slightly?
Many textbooks use π ≈ 3.14 or 22/7. The calculator shows both the exact answer and the 3.14 version.
What is the circumference of the Earth?
About 24,901 miles (40,075 km) around the equator, which is about 2π times the Earth's equatorial radius of 3,963 miles.
How do I measure the circumference of a pipe or tree?
Wrap a flexible tape measure or a string around it, then measure the string. Enter that as the circumference to get the diameter.
What is an arc length?
The length of part of the circle. A half circle is πr and a quarter circle is πr ÷ 2. Any arc is the circumference × angle ÷ 360°.
Does the unit matter?
Only for the label. The circumference comes out in the same unit as your input, and the area in square units.
What is π?
Pi is the ratio of any circle's circumference to its diameter, about 3.14159. It is the same for every circle, large or small.
How do I find the diameter from the circumference?
Divide the circumference by π. A circumference of 31.42 inches gives a diameter of 10 inches.
How big is a 12-inch pizza around the edge?
A 12-inch pizza has a circumference of about 37.7 inches and an area of about 113 square inches.
Built and reviewed by Hamza Farooq, founder and lead developer
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