Circle Calculator

This calculator lets you enter any one of a circle's four basic measurements — radius, diameter, circumference, or area — and works backward to recover the radius before computing every other figure from that single number. Diameter divides by two to reach the radius; circumference divides by two pi; area is inverted through a square root, since area equals pi times radius squared. Once the radius is known, everything downstream, including Area and Circumference themselves, is recalculated fresh from it rather than simply echoing whatever value was typed in, so switching Known Value from Area to Radius and back produces the same set of numbers either way, modulo the rounding each field applies.

The Sector Angle field controls two outputs only — Sector Area and Arc Length, both scaled down from the full circle's totals by the fraction the angle represents of 360 degrees — and it has no bearing whatsoever on the whole-circle Area, Circumference, Radius, or Diameter figures; a sector angle of 10 degrees and one of 350 degrees produce identical values for those four outputs while the sector-specific pair scales dramatically. This is a genuinely closed system: nothing here handles ellipses or any shape other than a true circle, and the tool does not account for a partial circle described any way other than through the Sector Angle field, such as a chord length or a segment area.