Everything about a circle follows from its radius r: diameter = 2r, circumference = 2πr and area = πr². Going backwards, r = circumference / (2π) and r = √(area / π). Pick which measurement you have and the calculator fills in the rest.
Example: a circle with radius 5 has diameter 10, circumference 31.4159 and area 78.5398.
π = 3.14159265... is the ratio of any circle's circumference to its diameter. The calculator uses the full-precision value from your browser.
Any. Lengths come out in the unit you typed and area in that unit squared (cm gives cm²).
Halve the diameter to get the radius, then use πr². Choose "diameter" above and it does this for you.
See also: Pythagorean theorem calculator · volume calculator · square footage calculator