If you need to find the diameter of a circle, many times you can do so with a tape measure. A circle is defined as a curved line in which every point is equally distant from the center. The diameter of a circle is the distance from a point on one side of the circle to a point on the other side going through the center. It also is expressed as twice the radius, which is a line measured from the center to any point on the circle.
Find the center of a circular area and mark it in some way.
Secure a tape measure anywhere on the circle and pull the tape measure to a point on the opposite side of the circle. Make sure the tape intersects with the center of the circle.
Measure just the radius of the circle if it is very large. The radius is the distance from the center to any point on the circle. Multiply the radius by two to produce a measurement for the diameter.
Measure the circumference (the length around the circle) of a circle that does not lend itself to measuring the diameter outright--something such as a pipe or a rod for which you cannot access the end to measure the diameter.
Determine the diameter from the circumference by dividing the circumference by the value of pi, which is 3.141592. Pi is the ratio between the circumference and the diameter and when it is divided into the circumference the result is the diameter.