It depends on what other information you have: area, circumference, radius, length of arc subtending a known angle, measure of angle for a known arc length etc.
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the equation for the circumference of a circle is 2*radius*pi, or since the radius is half of the diameter 2*radius=diameter, we can simplify the equation to the circumference of a circle=diameter*pi
The radius of a circle = the diameter of the circle divided by 2
Well, the equation for finding the circumfrence of a circle is (diameter)(pi). That is diameter times pi. since the diameter of a circle is 2 radii, we can say that there is 2(pi) radii in a circle.
The equation for determining the circumference of a circle is circumference = pi x diameter. So if the diameter is 5, then the circumference is pi * 5 = 15.71
circumference of a circle = diameter*pi or 2*pi*radius