Circumference Calculator

Worked check

With radius 3, C = 2πr gives about 18.85. With diameter 6, C = πd gives the same result. If those two routes disagree, the most common mistake is entering a diameter in a radius field or mixing measurement units.

Rule: C = 2πr = πd. Radius is half the diameter; select the correct input type before calculating. For a final check, estimate the expected size of the answer before copying it, keep units consistent, and round only the displayed result rather than intermediate values. Re-enter one simple case by hand when possible; a result with the wrong sign or order of magnitude usually points to an input or reference-base mistake.