Formula and method
The circle identities are d = 2r, C = 2πr and A = πr². Recover the radius from diameter by dividing by 2, circumference by dividing by 2π, or area by taking √(A/π). All other measurements follow from that radius.
Worked example
A circle of radius 5 m has diameter 10 m, circumference 31.41592654 m and area 78.53981634 m². An area of 100π m² instead gives radius 10 m.
Inputs and limitations
Choose an area unit when the known measurement is area. The result uses meters and square meters. A diameter used as a radius doubles circumference and quadruples area; check which measurement you have before calculating.