Circle Calculator

1. Choose which measurement you know. Click Radius, Diameter, Circumference, or Area.
2. Enter the value in the input field.
3. Read all four properties instantly: radius, diameter, circumference, and area are all calculated and shown.

Units are not specified because the formulas work for any unit. If you enter radius in centimeters, circumference and area are in centimeters and square centimeters respectively.

Given radius r:
  Diameter      d = 2r
  Circumference C = 2πr  (π ≈ 3.14159265)
  Area          A = πr²

Solving for radius from other measurements:
  From diameter:       r = d / 2
  From circumference:  r = C / (2π)
  From area:           r = √(A / π)

Example with radius = 5:
  Diameter      = 2 × 5        = 10
  Circumference = 2 × π × 5  ≈ 31.4159
  Area          = π × 5²     ≈ 78.5398