Circle Calculator
One known value is enough; everything else follows from π.
Radius versus diameter
Half of all circle mistakes are a factor of two: the diameter is 2r, and the area uses r — so doubling the radius quadruples the area.
Practical use
Pipes, tanks, round tables: measure the circumference with a tape and the calculator gives the diameter and area without a centre mark.
Is π exactly 3.14?
No — π is irrational. This uses the full double-precision value.
How do I find the area from circumference?
Area = C² / 4π.