There are a number of different standards for differnet types of waveguide. These tend to be country specific. Some of the major standards include:

WR waveguide system: EIA designation (Standard US) using a WR designator (Waveguide Rectangular) to indicate the size. The designator for a size consists of the letters WR followed by numerals the approximate narrow-wall dimension in hundredths of an inch. For example, WR90 has a narrow-wall dimension of 0.9 inches (approximately).

WG waveguide system: RCSC Designation (Standard UK). The waveguides types are given designators which comprise the letters WG followed by one or two numerals, e.g. WG10.