Calculator Use. Use this circle calculator to find the area, circumference, radius or diameter of a circle. Given any one variable A, C, r or d of a circle you can calculate the other three unknowns. Units: Note that units of length are shown for convenience. They do not affect the calculations.. What is the circumference of the circle with a 6 inches radius? The circumference of a circle equals the diameter times pi. The diameter is two times the radius. So a circle with a radius of 6 would have a diameter of 12, and a circumference of 12pi. The circle calculator finds the area, radius, diameter and circumference of a circle labeled as a, r, d and c respectively. For those having difficulty using formulas manually to find the area, circumference, radius and diameter of a circle, this circle calculator is just for you. ... If r = 6 cm, the the circumference is c = 2. "/>Find the circumference of a circle whose radius is 6 cm