SteelBWG, SWG, millimetres and inches
BWG is the Birmingham wire gauge and SWG is the Standard wire gauge, defined in the British standard BS 3737. In both, a higher gauge number means a thinner wire. The two systems do not agree: for the same gauge number, BWG and SWG assign slightly different diameters, as the table on this page shows column by column.
Inches are converted from millimetres by dividing by 25.4. The table gives both units for every gauge it lists, from gauge 4 to gauge 22.
Always write the diameter in millimetres
Because one gauge number can mean different diameters, give the actual diameter in millimetres or inches next to any gauge number, and name the system (BWG or SWG). A quote request that says only “gauge 12” leaves room for error. A request that says “BWG 12, with the diameter in mm” does not.
AWG is a different system
AWG, the American wire gauge, is used for round, solid, nonferrous, electrically conducting wire, and its dimensions are given in ASTM B258. It is mainly an electrical wire system. SWG has a similar but not interchangeable numbering scheme to AWG, so an AWG number must never be looked up in the BWG or SWG columns. Steel wire on this site is described by its diameter in millimetres.
How to use the table
- Find the gauge number in the first column.
- Read the column for the system you were given, BWG or SWG, in mm or in inches.
- If your gauge is not listed, ask for the diameter in millimetres instead of converting from memory.
