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.

### Step 1

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

### Step 2

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

4 * 4

= 16

### Step 3

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.