Question
Water Pressure ~65
1 inch water line to Sprinkler valve w/ 1 inch valve
Water flow ~10gpm
I want to use Hunter I-40 sprinkler head w/1 inch inlet. What is the formula to figure out how many heads I can put on one valve?  And is there other criteria to know prior to using the formula..  Thanks, Jay.

Answer
Thanks for your question.
If you look at a hunter I-40 nozzle chart the lowest one will operate is about 10GPM. Your valve will flow 25-30 GPM, but your 1" pipe is going to max out about 15-18 GPM. (you can push more through 1", but the pressure drop increases drastically.)
So the most you can run is 1 head per valve. If you were using a different sprinklers with 4GPM nozzles you could run 4 on the 1" without problems. So you see, in this case your pipe is the limiting factor.
Hope this answers your question.

Matt