Welcome to the world of fluid dynamics.

There are nomographs with pressure plotted against flow and there are charts showing pressure drop for pipes which the pipe manufacturer publishes.

Find pressure drop in pipe at volume you need and then pick a pump.

Thanks Willard. I kept trying to set it up as a physics problem working with pressure & pipe volume but could not get my units to cancel. I will look at the nomographs & go from there.

Bacsi,
Just what are you trying to do??????????/
Without getting too complicated depending upon the height you need to lift and the flow you wish to acheive is important. Sometimes people will try to over- commit you but with experience you'll find that the bare essentials are enough.
Hydroponic's is a lot of fun but can easily become an expensive hobby if you accept information which makes it more than it has to be.
Take it from one who's been doing it for a long long time.
Kind regards. Rosco

Hi Rosco. I was trying to figure out a pump size for someone else. You are an aeroponics grower, aren't you? The system was:
have 2 small barrells 10 round and 16-18 in long.
each will hose 3 plants and a 14in bubble stone each will a 120gph pump service
this system with 2 sprayers or drips per container running around 36 in of tubing 1\2?
What size pump is needed? (I don't have the lift height on the pump).
Any suggestions? Thanks.

In an open system like hydroponics, lift height is the most important number.

Pressure drop from 36" of 1/2" piping will be small enough to ignore at low pump volume.

Pumps will be 120gph at lift height.