European metric rebar sizes: Metric bar designations represent the nominal bar diameter in millimeters. Bars in Europe will be specified to comply with the standard EN 10080, although various national standards still remain in force.

U.S. Imperial Rebar Standard sizes: Imperial bar designations represent the bar diameter in fractions of 1⁄8 inch, such that #8 = 8⁄8 inch = 1 inch diameter. Area = (bar size/9)2 such that area of #8 = (8/9)2 = 0.79 in2. This applies to #8 bars and smaller. Bars higher in number have a slightly larger diameter than the one computed using the 1⁄8 inch formula

U.S. Imperial sizes: Imperial bar designations represent the bar diameter in fractions of 1⁄8 inch, such that #8 = 8⁄8 inch = 1 inch diameter. Area = (bar size/9)2 such that area of #8 = (8/9)2 = 0.79 in2. This applies to #8 bars and smaller. Bars higher in number have a slightly larger diameter than the one computed using the 1⁄8 inch formula.