Circumference Calculator
Find the circumference of a circle from its radius — instantly, using C = 2 · π · r.
Formula
C = 2 · π · r
r is the radius of the circle, π ≈ 3.14159.
About the circumference calculator
The circumference is the perimeter of a circle — the length of the closed line that bounds it. It is proportional to the radius and is found with C = 2 · π · r.
The radius is the distance from the center to any point on the circle. If you know the diameter, you can also use C = π · d, since the diameter is twice the radius.
The calculation runs instantly right in your browser — nothing is sent to a server, so everything stays local and private.
Benefits
Instant results
The circumference updates the moment you enter the radius.
Exact π value
Uses a precise value of π for a reliable result.
Fully private
All math happens locally in your browser with no uploads.
Frequently asked questions
How do I find the circumference from the radius?
Multiply the radius by 2 and by π: C = 2 · π · r. The calculator does this automatically.
How is circumference different from area?
Circumference is the length of the boundary (in linear units), while area is the size of the region inside (in square units).
Can I use the diameter instead?
Yes, with the diameter the formula is simply C = π · d. To use this calculator, divide the diameter by two to get the radius.
What unit will the result be in?
The circumference uses the same linear unit as the radius: if the radius is in centimeters, the circumference is in centimeters (cm).
Which value of π is used?
The calculator uses a precise value of π (≈3.14159) so the result is as accurate as possible.