Hi, I'm designig a Sprinkler system that fits a low pressure water system. My water pipe before my meter is 1 inch after the meter it's 1/2 inch. Now I want to design my system with a 1 inch main line and 1 1/4 inch lateral lines(Cheaper, and Complies with City Municipal Code). According to irrigation websites 1 1/4 inch poly pipe should give me 32 GPM and I plan on installing 12 sprinkler heads most of them running at 1.89 GPM. Can this be done and will it run effectively extra GPM is no problem to me this is just an estimate.
I don't think the system can work as you have described it.
Since you are the "designer" of the system, YOU are responsible for determining requirements and the capability of the source.
Design consists of developing a concept, and then analyzing the design concept to see if it will work. If it doesn't work, you need to change something about the design.
What is total GPM for the maximum-flow irrigation zone? 12 heads at 1.89 GPM per head is about 22.68 GPM.
What pressure do you need for the heads? Your main and laterals will have some pressure loss. Add that to the head pressure requirements.
NOTE: What is the rationale for putting 1 1/4" laterals on a 1" main? Seems bass-ackward.
Now, MEASURE the pressure that can be delivered to the main line through the meter and the line feeding the MAIN from the meter while MEASURING the maximum flow demand.
Based on what you have told us (1/2" line on the output side of the meter; 12 heads at 1.89 GPM per head; low pressure water system) the chance that you will be able to deliver the required flow is somewhere between slim and none.
IF your tests show that the supply won't deliver the flow and pressure that you need, then you must increase the supply or reduce the demand by reducing the number or flow rate of the sprinkler heads. That is called "iterating the design". After you come up with a new design, you analyze that new design to see if it will work.
You say this is only for an estimate. The fact that is is "just an estimate" will not mean much if you build it and all of the sprinkler heads just dribble out onto the grass.