The diameter of a circle is the length of a straight line going from one side of a circle to the opposite side. To measure the diameter, simply choose two points on the edge of a circle that are directly across from each other, and measure the distance between these points. You will need a measuring device to perform this task.

**Obtain a ruler or other measuring device**The first step in measuring the diameter of a circle is to gather a measuring device, such as a ruler, meter stick or yard stick. The measuring device should be larger than the diameter of the circle to obtain precise measurements.

**Place the measuring device on any point on the edge of the circle**Line up the point on the edge of the circle with the first notch on the ruler or measuring device.

**Extend the measuring device across the circle**Extend the measuring device to the opposite end of the circle, making sure that it's straight across from the original point crossing the center. The diameter of the circle is the distance between the first notch on the ruler or measuring device and the notch that comes into contact with the circle's edge on the other side.