Find a circle’s circumference fast. Enter the radius, diameter, area or circumference and this free calculator solves all the rest using C = 2πr = πd and A = πr².
Enter any one value
Fill in any one of radius, diameter, circumference or area, and the calculator works out the rest. Clear a field to enter a different one.
Circumference
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Enter one value
Radius
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Diameter
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Circumference
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Area
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Circumference is the distance around a circle: C = 2πr = πd. The area is A = πr². Results use units consistent with your input (e.g. cm in → cm out, cm² for area).
Use this free circumference calculator to find the circumference of a circle. Enter the radius, diameter, area or even the circumference itself, and it works out all the other values instantly.
What this circumference calculator does
A circumference calculator finds the distance around a circle. Enter any one of radius, diameter, circumference or area, and it solves for the rest — using C = 2πr = πd and A = πr².
How to calculate circumference
C = 2 × π × r (from radius)
C = π × d (from diameter)
A circle with a radius of 5 has a circumference of about 31.42 and an area of about 78.54. Double the radius gives the diameter (10).
Circle formulas at a glance
Quantity
Formula
Diameter
d = 2r
Circumference
C = 2πr = πd
Area
A = πr²
Radius from area
r = √(A ÷ π)
What is π (pi)?
π ≈ 3.14159 is the ratio of any circle’s circumference to its diameter. It’s constant for every circle, which is why these formulas work whatever the size.
How to use the circumference calculator
Enter any one value — radius, diameter, circumference or area.
Read the circumference at the top.
See all the other values below.
Circumference FAQ
How do I calculate circumference?
Multiply the diameter by π, or the radius by 2π: C = πd = 2πr. A circle with radius 5 has a circumference of about 31.42.
How do I find circumference from the diameter?
Multiply the diameter by π (about 3.14159). A diameter of 10 gives a circumference of about 31.42.
How do I find the radius from the circumference?
Divide the circumference by 2π. The calculator does this automatically — just enter the circumference and it returns the radius, diameter and area.
What's the difference between circumference and area?
Circumference is the distance around the circle (C = 2πr); area is the space inside it (A = πr²). This tool shows both.
What units does it use?
Whatever unit you enter — the circumference and diameter come out in the same unit, and the area in square units. Just keep your input consistent.
Is the circumference calculator free?
Yes, this circumference calculator is completely free, needs no sign-up, and gives instant results in your browser.
