Convert the measure of the arc angle to radians by dividing the number of degrees by 57.3. Each radian equals 57.3 degrees. For example, if the angle measures 78 degrees, divide 78 by 57.3 to get 1.36 radians.
Divide the arc length by the angle measure in radians to find the circle radius. In this example, if the arc is 5.1 inches, divide 5.1 by 1.36 to get 3.75 inches as the radius.
Multiply the radius by 2 to find the circle diameter. In this example, multiply 3.75 by 2 to get 7.5 inches as the diameter.
Multiply the diameter by pi to find the circle circumference. In this example, multiply 7.5 by 3.14 to find the circumference of the circle equals 23.55 inches.