View Full Version : Calculating Pump Flow

04-03-2005, 10:32 AM
I've gotten myself a little confused about calculating pump flow. For example, I have a Mag12 that is rated for 1200GPH that I would like to split into 3 returns. If each of those returns is 1/2" (to total the 1 1/2" recommendation for this pump) will each of those 1/2" returns get 1200GPH or will it be reduced by 1/3 so each return is getting aprox 400GPH?


04-03-2005, 11:09 AM
I may be wrong, my understanding is that a mag 12 puts out 1200 gph at 0' head. At zero head you would have 1/3 of 1200 out of each outlet.

04-03-2005, 12:23 PM
Sounds right to me, the pump is only going to do 1200 gph therefore if you split it 3 ways you will get 400 gph at each outlet. And thats at 0 head.

A Mag12 at 4' head which is pretty close to norm if coming from sub is only going to be 1130, which would equal about 377 gph at each outlet.

Hope that helps

04-03-2005, 01:22 PM
One correction: 3 1/2" outlets is not equal to one 1.5".

Rule of Thumb:
2 - 1/2" outlets = 1 - 3/4"
2 - 3/4" outlets = 1 - 1"
2 - 1" outlets = 1 - 1.5"

Also keep in mind it is difficult to get equal amounts of flow out of split outlets unless you are pushing the maximum of that diameter of pipe. Water finds the easiets way out and so if each outlet even has a small difference in head preasure there will be an uneven distribution.

Your plan could still work for your application, I just want to be sure you have clear expectations.

04-04-2005, 12:13 AM
The other thing you left out is the number elbows and the friction of the pipe. If you run 3 exits from this pump, then I bet the flow from each pump is probably more in the neighborhood of 250-300gph. Really the mag is not the best pump to work in this situation, unfortunately.