How Do You Find the Diameter of a Circle?
Find the diameter of a circle with a given circumference by solving for the formula “d=C/pi,” where d is the diameter, C is the circumference and pi is 3.1459. For this problem, find the diameter of a circle that has a circumference of 18.8754 inches.

Input the variables of the equation
Replace the equation d=C/pi by switching C with 18.8754 and substituting 3.1459 for pi. This gives the equation d=18.8754/3.1459, or the diameter is 18.8754 divided by 3.1459.

Solve the equation
Divide the numbers to discover the diameter of this particular circle is 6 inches. Check your answer by inputting the numbers for the equation C=pi*d, or the circumference equals to the diameter times pi, and determine that six times 3.1459 is 18.8754.

Find the center and measure
When you do not have the circumference of a circle to use in the equation, find the center of a circle using graphing paper and a protractor. Draw a straight line between two opposite points on the graph to get a straight line through the center. Measure this straight line with a ruler as a diameter.

Use the radius
Use the radius of a circle to determine the diameter of a circle. The diameter is simply twice the radius since a radius is a straight line drawn from the center of a circle to the edge of a circle, and a diameter is a line through two opposite points of a circle.