 HagenPoiseuille equation

 
Note that you only need to enter either the velocity or the discharge rate (wichever you know). Also, be sure only to use this calculation for appropriate cases. For other conditions, other suitable calcs are available. This equation assumes that all fluid flow is continually along the pipe, with the center moving fastest and the outside slowly (because of drag from the walls). Thus turbulent effects (appearing at ~Re=2000) are not taken into account.  