Calculating C-channel beams is a relatively simple procedure, as C-channel designs have been simplified into just two dozen types. This is due to the realities of steel manufacturing, which cannot create custom designs for every work order, and because standardization of C-channels makes steel-based construction projects that much simpler. You will need calipers and a measuring tape to determine the dimensions of a C-channel beam to compare to a code sheet. This sheet will then give you all the values you need without having to do much math.
Things You'll Need
Standard C-channel code sheet
Measure the depth of the C-channel beam by placing a measuring tape on the flat long side of the beam in the center of the C. Even if the side is laying flat and is not vertical, the middle part of the C is still referred to as the height of the C-channel in order to simplify identification. Write down that measurement in inches.
Measure the width of the flanges. The flanges are the top and bottom of the C and are always referred to as the width measurement, even if they are not horizontal. Write this measurement down in inches.
Measure the height of the flanges by measuring from the inner valley of the C to the tops of the flange. Subtract this measurement from the width measurement to get the web thickness value in inches.
Consult the C-channel code sheet. The C-channel will first be separated by different depths, then by weight; since weighing a C-channel beam is impractical, match the web thickness values, since those are unique to each weight. Once you have located the C-channel type you have, the sheet will give you the moment of inertia and section modulus for that C-channel beam.