DIN EN 1506

Usual Diameters (Ø)

Unless otherwise specified the diameters always refer to the inner diameters of the ducts and fittings.

The illustration below shows which rond duct connections can be used
depending on the duct diameters:

MU Overcollars
PF Flanges

AF Flanges
LF Flanges
BF Flanges
UF Flanges

METU-FORM Ducts, Fittings and Connections