All 3 sizes of Hudson Valves will put 80% of the water supplied to it through it. To get an accurate flow rate for your application you can use a bucket and timer. Start your timer and turn your water source on full volume. When the bucket is full stop your timer. To find gallons per minute you will divide your bucket size by the time it took to fill your bucket and multiply by 60.  This is the gpm at your water supply. When the valve is operating, it will put 80% of that gpm rate through it.

GPM = container size in gallons
              Time to fill container

For example:  a 2-gallon bucket took 45.6 seconds to fill

2 divided by 45.6 = 0.044
0.044 x 60 = 2.64 gallons per minute

Calculating 80% of gpm: 2.64 x .80 = 2.11
2.11 gallons per minute when valve is in operation.