Identify the center of the circle, and draw a line that passes through the center point and cuts the circle on either end. This line is the diameter of the circle. If the center of the circle is not marked, draw any line across the circle so it cuts the circle at two points. It is not important where you choose to draw this line. Use the ruler to measure the length of this arbitrary line, called a chord, inside the limits of the circle. Divide the length in half to find the midpoint of the chord and mark this midpoint on the line with the pencil. Position the ruler at the midpoint marking and align the ruler so that it is at right angles to the line. Draw a second line that passes through the pencil marking and extends in both directions until it cuts the circle. This second line is the diameter of the circle.
Measure the length of the diameter using the ruler.
Multiply the diameter by the mathematical constant pi, the value of which is approximately 3.14. If a higher degree of accuracy is required, use the value of the pi button found on all scientific calculators. The product of the diameter and pi is the circumference of the circle.
Report the distance around the circle using the same units used to measure the diameter.