Irrigation Zone Calculator

Zone Area, Head Spacing, and GPM per Head are the only three values actually used in the math — heads needed comes from area divided by spacing squared, zone flow is heads times GPM per head, and run time is derived from those. Water Pressure and Head Type are captured as context to help you pick a realistic GPM per head (spray heads typically run 1-2 GPM, rotors 2-6 GPM), but they aren't plugged into any formula, so changing them alone won't move the outputs.

It divides your zone area by the head spacing squared (spacing × spacing) and rounds up, which models heads laid out on a simple square grid. That's a reasonable starting estimate, but some designers use triangular spacing or head-to-head overlap patterns instead, which can call for a different head count than a straight square-grid calculation.

It's the minutes needed to deliver 1 inch of water across the zone in a week, based on the fact that 1 inch of water over 1 square foot equals 0.623 gallons. The calculator multiplies your zone area by 0.623 to get weekly gallons needed, then divides by zone GPM to get minutes — useful for setting a controller, but the 1-inch/week target itself is a general assumption, not tailored to your specific plant type, soil, or local evapotranspiration.

The pipe size is a simple lookup based only on zone flow rate — under 5 GPM suggests 3/4 inch, up to 12 GPM suggests 1 inch, up to 22 GPM suggests 1.25 inch, and above that 1.5 inch. It doesn't factor in pipe run length or elevation change, both of which add real friction loss and can require a larger pipe than the flow-rate lookup alone would suggest.