The formula
Circumference is pi times the diameter, written C = pi x d. Because the diameter is twice the radius, it is also C = 2 x pi x r.
Pi is the same for every circle: about 3.14159. The calculator uses it at full precision and rounds only the answer it shows.
Worked example
A circle with a radius of 5 m has a diameter of 10 m. Its circumference is 2 x pi x 5 = 31.415927 m.
Its area is pi x 5 squared = 78.539816 m². Both come from the same radius, so changing one measurement changes the other.
Circumference or area
Circumference is a length, measured in metres, centimetres or inches. Area is a surface, measured in square units.
If you know the circumference and want the area, enter that circumference and read the area row. The radius is the circumference divided by 2 pi.
Inputs for the circumference calculator
Measurement you know
Choose radius, diameter, circumference or area. Changing this keeps the same circle.
Measurement unit
The unit of the measurement you typed. The answer is given in metres, with your own unit alongside.
Measurement
Enter a positive number in the selected unit. Keep the precision you measured; the calculator rounds only when showing the result.
CALCULATOR QUESTIONS
Circumference calculator questions
How do I find the circumference from the diameter?
Multiply the diameter by pi. A 10 m diameter gives 31.415927 m.
How do I find the radius from the circumference?
Divide the circumference by 2 pi. A circumference of 31.415927 m gives a radius of 5 m.
Why is circumference measured in metres and area in square metres?
Circumference is a distance around a line, so it has one dimension. Area covers a surface, so it has two.
Does the formula differ between countries?
No. This is pure geometry, and the imperial units used here are the international definitions.