Circle Circumference Calculator
Calculate the circumference (perimeter) of a circle from its radius or diameter. Formula: C = 2 x pi x r. Instant, accurate results.
How to use this tool
- Enter radius in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your circumference and the full breakdown beneath it.
Formula
Circumference = 2 x pi x r (or pi x diameter)
How it works
Multiply the radius by 2 and then by pi to get the circumference. Equivalently, multiply the diameter by pi.
Worked example
Circle with radius 5 m
- Circumference = 2 x pi x 5
- Circumference = 10 x pi
- Circumference = 31.4159 m
31.4159 m
Common mistakes to avoid
- Confusing radius and diameter and entering the full width; if you measured across the circle, use the diameter input rather than halving manually.
- Reporting the area (pi x r^2) when asked for the circumference (2 x pi x r), or vice versa.
- Using an approximate value of pi (3.14) for precision work, introducing rounding error.
Key terms
- What is circumference?
- The circumference is the total length around the outside of a circle.
Frequently asked questions
- I know the circumference. How do I find the radius?
- Rearrange: r = C / (2 x pi).
- What is the difference between circumference and arc length?
- Circumference is the full perimeter of the circle. Arc length is the perimeter of only a portion, defined by a central angle.
- Can I use this for a circular track to find the distance of one lap?
- Yes. Measure the radius of the track center line and compute its circumference. One lap equals that circumference.