How to Use the Circumference Calculator

Choose Radius or Diameter, enter the value, and the circumference — along with the other measurement — fills in below.

circumference = 2 × π × radius
circumference = π × diameter

Both formulas say the same thing, since a diameter is just twice the radius — π itself is defined as the ratio of any circle's circumference to its diameter, so this calculator is really just applying that definition directly rather than deriving anything new.