Circle Calculator

Formula

How this is calculated

A circle is defined entirely by one measurement: its radius r, the straight-line distance from the centre to the edge. Every other property follows from r and the constant π (pi ≈ 3.14159), the fixed ratio of any circle's circumference to its diameter. The calculator takes the radius you enter and derives the rest geometrically, so the answers are exact in terms of π rather than approximations.

The diameter is simply twice the radius: d = 2r, since it spans the circle through the centre. The circumference — the distance around the edge — is C = 2πr, the diameter multiplied by π. The area enclosed is A = πr², so doubling the radius quadruples the area because r is squared.

All inputs and outputs use one consistent unit you pick (cm, m, inches, etc.): length results share that unit, while area is in square units. The radius should be a positive number; a zero or negative radius has no real geometric meaning. Results are rounded to four decimal places, so values involving π are shown approximately.