Circle Circumference Calculator

Calculate the circumference of a circle using its radius or diameter. Simply enter your measurement, choose the unit, and get instant results.

How to Use This Calculator

  1. Choose whether you want to input the radius or diameter
  2. Enter your measurement value
  3. Select your preferred unit (mm, cm, m, km, in, ft, yd, or mi)
  4. Click "Calculate" to get the circumference

Formula: Circumference = 2 × π × radius or Circumference = π × diameter

Worked example

A circle with radius = 7 cm:

If you'd entered 14 as a diameter instead of 7 as a radius, the tool divides by 2 internally first, so the results above are identical either way.

How the rounding is chosen

The displayed values aren't fixed at two decimal places. Once the circumference drops under 1 unit, the tool switches to 3 decimals; under 0.1 it uses 4; under 0.01 it uses 6. That keeps very small circles (say, a 0.05 mm radius washer) from displaying as "0.00" instead of a usable number.

Input requirements

Only positive numbers are accepted — zero or negative radius/diameter values return an error, since a circle can't have negative size. Unit conversion is not performed; the unit you pick from the dropdown is simply printed next to the result, so mixing cm input with an "in" label won't convert anything.