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I have 3/4" pipe coming into my house, but every faucet in my house including the hose connections outside uses 1/2" pipe. I would like to tap into the 3/4" pipe for the sprinkler system since the flow rate that I measured out of the 1/2" hose connection only provides 8.89 gpm, which is barely above the minimum 8 gpm required by RainBird. The problem is that I don't have anywhere to measure the flow rate out of the 3/4" pipe.

Seems like there has got to be a way to estimate the flow rate of the 3/4" pipe, knowing the flow rate, velocity and pressure of a 1/2" pipe in the same house.

Pressure: 58 PSI

Velocity @ 1/2": 14.71 Feet Per Second

Flow Rate @ 1/2": 8.89 GPM

I can think of two ways to estimate this, but I don't know if they are even in the ballpark or not.

#1: since 3/4" pipe is about 67% larger than 1/2" pipe, I could multiply 8.89 * 1.67 to get roughly 14.8 GPM. This seems to neglect too much to be even roughly accurate, but I could be wrong.

#2: use a flow rate calculator, such as:

http://www.1728.org/flowrate.htm

with 3/4" pipe and the same 14.71 FPM velocity that I calculated with the 1/2" pipe. The problem with #2 is that I don't know if velocity is constant across multiple pipe sizes at the same height even though pressure is constant at the same height. The GPM calculation with method #2 comes out to 20.3 GPM.

Any thoughts?

Thanks,

Adrian