NBN EN ISO 13351:2009

This European Standard specifies size designations for industrial fans and specifies dimensions for the circular and rectangular flanges of
general purpose industrial fans as defined in 3.1.1. It does not apply to cross-flow fans, or fan appliances used for household or similar
applications.
For circular flanges, this International Standard provides two different flange series, one for standard casing thicknesses and a second for heavy duty fans as used on board sea-going vessels or in heavy industry. In order not to restrict fan designs unduly, only the pitch diameter, hole numbers and hole diameters are standardized. Flange thickness as well as internal and external flange diameters can be chosen freely within the limits of good engineering practice.