Example 1
- Radius: 10
62.83
r = 10 gives 62.83 of circumference.
Circumference calculator
Enter the radius. The calculator computes circumference 2πr and area πr². At r = 10 the circumference is about 62.83 and the area is about 314.16.
C = 2πr. Area is on the area of a circle calculator.
Result
Radius r (cm). Circumference shows up here.
How it works
Circle circumference is two π times the radius. At r = 10 you get 62.83. That is tape around the disk, not paint on the disk. Area at the same 10 is 314.16 and is on the neighbouring page.
One gives 6.28, because a two sits in front. Five gives 31.42. Two gives 12.57, the same digits as area at r = 2, another formula, a coincidence.
If you know the diameter, divide by two before you type r. πd at d = 10 is the same 31.42 as 2πr at r = 5. Typing 10 as a diameter doubles the hoop. The calculator does not take a diameter. Sphere 4πr² at r = 10 is about 1256.64, not this page.
The calculator does not run when the radius is missing. Switching to inches relabels the result as in and does not convert a typed 10. The calculator uses π. A half gives 3.14, because 2π times a half is π.
Seven gives 43.98. Twelve gives 75.40. Three gives 18.85.
Pythagoras does not wrap a circle: there are no sides a and b here, only one radius. The result is a length, not an angle in radians. If you want how much paint, open circle area with the same r.
How to use
- Type the radius, not the diameter, in cm or inches from the unit switch.
- Click Calculate and read 2πr to two places with a length unit.
- Do not look for area here: r = 10 is 62.83 of circumference, and area 314.16 is on the sibling card.
- If you know the diameter, divide it by two and only then type that result as r.
- The r = 5 example is 31.42. That is not 78.54 from area and not 1256.64 from a sphere.
Formula
C = 2 × π × r
Not area πr².
C = 2πr at r = 10
Radius, not diameter. r = 10 gives 62.83 of circumference. r = 1 gives 6.28. Not area πr².
- C
- Circumference 2πr. r = 10 is 62.83. r = 5 is 31.42. r = 7 is 43.98.
- r
- Radius. 10, not 20. r = 2 gives 12.57. Disk area at the same r is on circle area.
- π
- Inside 2πr. At r = 1 the result is 6.28, about 2π. At r = 10 that is 62.83, not 314.16 of area.
Examples
Example 2
- Radius: 1
6.28
r = 1 gives 6.28.
Example 3
- Radius: 5
31.42
r = 5 gives 31.42.
Example 4
- Radius: 2
12.57
r = 2 gives 12.57.
Example 5
- Radius: 7
43.98
r = 7 gives 43.98.
Example 6
- Radius: 0.5
3.14
r = 0.5 gives 3.14.
Example 7
- Radius: 12
75.40
r = 12 gives 75.40.
Example 8
- Radius: 3
18.85
r = 3 gives 18.85.
Frequently asked questions
Is this the same as circle area?
No. Circumference is 2πr. Area is πr². At r = 10 the circumference is 62.83 and the area is 314.16. Two cards, two formulas.
Why does r = 1 give 6.28, not 3.14?
Because of the two in front. 2 times π times 1. Bare π without the two would be area at r = 1, not circumference.
Can I type diameter 10 and get πd?
The code waits for r. πd at d = 10 is the same 31.42 as 2πr at r = 5. Type 5, not 10, if 10 is the diameter.
Is sphere 4πr² on this page?
No. Sphere surface has its own card. At r = 10 it is about 1256.64, not 62.83.
What if I leave r empty?
Nothing is computed. Type a radius or load the 10 example. Empty r is not a rim measurement.
Do inches multiply 62.83 differently than cm?
Not the digits. A typed 10 still computes 2π times 10. The in label changes. Convert yourself when 10 cm is not 10 in.
Why does r = 0.5 give 3.14?
Because 2π times a half is π. That is the same 3.14 as area at r = 1, another formula, another calculator.
Does Pythagoras wrap this circumference?
No. Pythagoras is the sum of squares of the legs. The circle in this calculator has no sides a and b, only one radius.
Is the result in radians?
No. It is a length. Radians live on the degrees-and-radians page, not here.
Is r = 3 equal to 18.85?
Yes. 6π rounds to 18.85. At r = 7 you get 43.98, at r = 12 you get 75.40.
Knowledge sources
The calculator computes the same formula as the definition below.
Page updated in 2026.