Sprinkler Calculator

Sprinklers are specified in gallons per minute, but watering is specified in inches. The bridge between them is one constant:

That 96.25 comes from 60 minutes per hour × 231 cubic inches per gallon ÷ 144 square inches per square foot. Once you know the precipitation rate in inches per hour, run time is just the depth you want divided by the rate.

The design rule is head-to-head coverage: every sprinkler's spray must reach the heads next to it. It sounds like massive overlap, and it is, but sprinkler output is not uniform across its radius. It applies far more water near the head than at the edge of its throw, and overlapping is what evens that out.

Spacing heads at their full radius gives even coverage on a square grid. A triangular grid, with alternate rows offset, can stretch to about 110% of the radius and uses roughly 15% fewer heads for the same area.

Wind changes this. Above about 5 mph, reduce spacing to 50–55% of the diameter of throw, or you get dry arcs downwind.

This is the single most common irrigation design mistake. A spray head applies water roughly three times faster than a rotor. Put both on the same valve and whatever run time you choose is wrong for one of them, either the spray zone floods or the rotor zone stays dry.

Group heads by precipitation rate, not by convenience of piping. Sun and shade areas are worth separating for the same reason.

If your system applies water faster than the soil can absorb it, the excess runs off, you paid for water that watered the pavement. Clay soil accepts roughly 0.15 in/hr, while a spray zone delivers 1.5 in/hr, so runoff starts within minutes.

The fix is to split the run into shorter cycles with soak time between them. Three 8-minute cycles an hour apart deliver the same water as one 24-minute run, with none of the runoff.