Circumference Calculator
Get the perimeter of any circle instantly. Enter the radius, diameter or even the area, and the calculator works out the circumference for you - with each step of the formula shown.
How to use
- Select whether the value is the radius, diameter or area.
- Enter the value.
- The circumference will show with steps.
Tips
- Circumference is just 2 times pi times the radius - easy to sanity-check the result.
- If you know the diameter, multiply by pi directly (about 3.14).
- Starting from area? The tool reverses the area formula for you - no square roots by hand.
- Keep units consistent: radius in meters gives circumference in meters.
Note: Formula: C = 2 pi r = pi x d. The ratio of circumference to diameter is always pi (3.14159...), no matter how big the circle is.
Frequently Asked Questions (FAQ)
What can I enter?
The radius, diameter or area. The circumference shows with steps.
Which formula is used?
C equals 2 pi r, which is the same as pi times d, with pi equal to 3.14159. The ratio of circumference to diameter is always pi, no matter how big the circle is.
Can I start from the area?
Yes. Select the area option and the tool reverses the area formula for you, so you never have to do the square root by hand.
How do I sanity-check the result?
Multiply the radius by about 6.28 (2 times pi). If the answer is close to that, the result is right.