European metric bar designations represent the nominal rebar diameter in millimetres. U.S. Imperial bar designations represent the bar diameter in fractions of 1/8 inch, such that #8 = 8/8 inch.

For determining beam spans (distance a beam can span between supporting posts), consider the following concept. The more weight, or the longer the supported length of joist that a beam must carry, the shorter the span of the beam – for a particular size of beam.