The diameter of a circle is the length of a straight line from one edge of the circle to the opposite edge, through the center point of the circle. The diameter is always the longest line that can be drawn from side to side. When two circles are drawn with the smaller circle inside the larger one, the inside diameter is the diameter of the smaller circle. The inside diameter of a metal pipe or other kinds of tubing is the distance from one inside edge to the opposite inside edge, crossing the center point. This mathematical concept has many practical applications for the home handyman.
Draw a circle on a sheet of paper using a pencil and compass to practice measuring the inside diameter of a two-dimensional circle. Outline the circle with a thick black marker. Draw a straight line through the center point of the circle with the pencil, starting at the inside edge of the black line on the circle and ending at the opposite edge of the circle on the inside edge of the thick black line. Note that this diameter is the longest possible line that can be drawn through the circle.
Align the "0" point of the ruler with the spot on the circle's edge that meets the straight line. Check the length of the line by examining the point on the ruler that touches the point on the opposite edge of the circle that meets the opposite end of the line, to determine the measurement of this inside diameter.
Align the "0" point of the ruler with one of the edges of the inner part of the three-dimensional tube to be measured. Hold this edge firmly with one hand while pivoting the ruler slightly up or down at the opposite edge of the tube, estimating visually where the center point of the inner circle is and having the top edge of the ruler touch that point.
Make note of the length of the distance on the ruler from the "0" point to the point where the top edge of the ruler touches the inner edge on the opposite side of the circle.
Pivot the ruler up a very small amount, about 1 mm. Make note of distance from the "0" point on the ruler to the point where the ruler touches the inner edge of the tube on the other side. Pivot the ruler down this same small amount and make note of this new measurement.
Repeat the process of moving the ruler slightly up and down and recording the various lengths as described in Step five until you are certain you have found a position for the ruler that results in the longest possible measurement from one side of the circle to the other. Make note of this measurement which is the length of the inside diameter of the tube.
The center point of the two dimensional circle can be determined by finding the slight indentation in the paper caused by the pointed end of the compass pressing into the paper.
You can also invest in a caliper if you will be measuring diameters often.
Verify the accuracy of your measurement by repeating the process two or three times to be sure it is correct or you risk purchasing materials that may not be the correct size.