Two rates describe how quickly water flows through a pipe:
 The volumetric flow rate describes the water's flow in terms of gallons or cubic meters per minute.
 The flow velocity describes the linear flow rate, measuring it in terms of feet or meters per minute.
The volumetric flow better describes the total quantity of water that flows, but the flow velocity gives a better idea of the water's actual speed. Use the pipe's dimensions to convert between the two rates.

Multiply the volumetric flow rate by 0.408, a conversion constant. For example, if, 600 gallons pass through the pipe each minute, you would perform the following operation: 600 x 0.408 = 244.8.

Find the square of the pipe's diameter. For example, if the pipe has a diameter of 6 inches, you would perform the following operation: 6^2 = 36.

Divide the answer to Step 1 by the answer to Step 2, as follows: 244.8 / 36 = 6.8. This is the water's flow velocity in feet per minute.
References
 Photo Credit Jupiterimages/Photos.com/Getty Images