This International Standard gives dimensional details of circular and rectangular flanges of fans in addition to specifying the fan size designation. For circular flanges, the values specified in ISO 6580 have been retained for light-duty fans, in parallel with those values given in Tables 3 and 4 for medium- and heavy-duty fans. While it does not constrain the manufacturer's choice of flange details, this International Standard facilitates interchangeability, thereby helping to reduce technical barriers to trade. Throughout this International Standard, the principal dimensions are based on the rounded values of preferred numbers given in ISO 497.