Choose the measurement you already know, enter a positive real number, and the calculator derives the rest of the circle values.

1. Select radius, diameter, circumference, or area.
2. Enter a positive value in the matching input.
3. Read the result card for all derived circle measurements.

With radius 5, the calculator returns diameter 10, circumference 31.415927, and area 78.539816.

That matches the formulas exactly: d = 2r, C = 2πr, and A = πr².

The calculator rejects zero, blank, malformed, and non-finite inputs so it cannot silently coerce bad values into a circle.

Negative lengths are not accepted because radius, diameter, circumference, and area are all non-negative geometric quantities.