The diameter of a circle is the measurement of a straight line going from one point on the edge of the circle, through the center, and to another point on the opposite edge of the circle. You can figure the diameter through a variety of methods, depending on the measurements that you do know. To calculate this, you may need to use the value pi. Pi is a mathematical constant that is an irregular number, usually represented as 3.141593.
Use a ruler to measure the diameter of a circle. If you are trying to find the diameter of an actual circle--as opposed to the theoretical ones in math problems--use a ruler to measure one end of the circle to the other, making sure that your ruler hits the exact center. If you're not sure where the center is, you can do your best to approximate a few times and then average your results.
Multiply by two if you know the radius. The radius is the measurement from the exact center of the circle to the outside line. It is exactly half of the diameter, so you simply need to double it to get the measurement for the radius. For example, if the radius is 5 cm, the diameter would be 10 cm.
Divide the circumference by pi if you know the circumference. This is the measurement of the outside of the circle. If the circumference is 21.98 cm, the diameter is 9 cm.
If you know the area, take the square root of four times the area divided by pi. For example, if the area is 78.5 cm, you'd first multiply that by four to get 314 cm. Then, divide that by pi to get 100, then take the square root of 100, which would mean that the diameter is 10 cm.