Different lengths are given for the Main. The reason is that different origins and thus headwaters are attributed to the river in each case. Together with the longer headwaters, the Roter Main, a flow length of 527 km results, together with the larger but shorter headwaters Weißer Main one of 518 km. From the confluence of these two headwaters, the Main has a length of 472 km. About 81 km downstream of the confluence the Regnitz enters, which is much larger than the Main there. If one considers the waterway from the course of the Regnitz together with its main headwaters Rednitz and again its own main headwaters Fränkische Rezat as a source river, the total length is even 553 km. Before German reunification, the Main was sometimes called the longest river flowing exclusively in the then Federal Republic.
The catchment area of the Main and its tributaries covers 27,292 km² and extends over most of Franconia, the north-easternmost part of Baden and the north of southern Hesse. It borders the Danube catchment area in the south (a few hundred metres south of the source of the Weißmain lies the source of the Fichtelnaab, which drains into the Black Sea via the Naab and Danube); the boundary between the two is part of the main European watershed.
With an average discharge of 211 m³ per second at its mouth into the Rhine, the Main is the fourth largest tributary of the Rhine after the Aare (560 m³/s), Meuse (357 m³/s) and Moselle (315 m³/s).
The 388 km long navigable stretch begins near Bamberg and has been connected to the Danube via the Main-Danube Canal since 1992. There are several large inland ports, especially in the Rhine-Main conurbation around Frankfurt.
The Main runs through the federal states of Bavaria and Hesse. Baden-Württemberg has a share of the left bank in the area of the towns of Freudenberg and Wertheim over a length of around 25 km.
The Main hiking trail and the Main cycle path run along the Main.
See also: List of cities and towns on the Main River