Plumbers usually measure flow rates in terms of the volume of water that passes each minute. Convert this value to velocity using the pipe's size.

Multiply flow rate in gallons per minute by 0.408. For instance, if the flow rate is 50 gal / min:

50 * 0.408

\= 20.4

Square the pipe's diameter, in inches. For instance, if the pipe measures 4 inches across:

4 * 4

\= 16

Divide the answer to step 1 by the answer to step 2.

20.4 / 16

\= 1.275

This answer is the water velocity in feet per minute.