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Tuesday, April 13th 2010, 7:44pm

Confused about GPM in books I picked up from the store

I followed the directions on a brochure and a sprinkler book to determine my static pressure and the GPM using a 5 gallon bucket. The results are as follows:

Static Pressure: 76

GPM at 40 PSI: 2.4

GPM at 45 PSI: 2.04

I just stopped there. The GPM are so low I would only be able to put 1 sprinkler head per zone with those numbers. After thinking about it a bit I see that the logic in the book and the brochure is completely asinine. You should be tying directly into the main line, not some 3/8 inch copper pipe which is what my faucets are. They tell you in this book to tie into the main line just five pages after telling you to determine your water capacity using your puny little faucet. So why are the people writing these brochures telling everyone to use their outside faucets to determine capacity? 3/8 vs 1 & ΒΌ is a big difference in throughput right? Have I gone off the reservation? Seems to me that the bigger the pipe the higher the pressure and the more GPM correct?


Supreme Member


Tuesday, April 13th 2010, 9:33pm

Yes the bucket tests are not accurate. Just read up on the subject at
But the bucket test is in these brocures because it is easy.


Active Member

Posts: 43

Location: east coast


Tuesday, April 13th 2010, 10:16pm

gpm and pipe size

Service line type and size along with static pressure is a start in determining your gpm capacity. You need to factor in such things as elevation and friction loss also. A bucket test is useful only if you draw the water from the pipe that is going to be your sprinkler water source . ( not a branched off hose bib)Never base a design solely on its results . Your test should have yielded a higher result at the higher pressure(45psi) I don't know what happened there , maybe you miscalculated???.
A larger pipe does not create more pressure, it just has less pressure loss through it at a given gpm than a smaller pipe .
Yes, there is a major difference in capacity between a 1/2" pipe and a 1" pipe.


Tuesday, April 13th 2010, 11:01pm

Awesome, just what I needed to confirm my thoughts. Thanks!

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