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Irrigation Calculator Guide
Run time from your actual sprinkler output, rather than guessing at minutes and hoping.
Precipitation rate (in/hr) = 96.25 × GPM ÷ (spacing A × spacing B)
Where does 96.25 come from? A gallon is 231 cubic inches. Spread over one square foot (144 sq in) that's 1.604 inches deep. Times 60 minutes = 96.25. It's not a magic number — it's unit conversion.
3 GPM heads at 15×15ft → 1.28 in/hr → about 47 minutes for an inch.
Spacing Drives the Rate, Not Head Count
The rate depends on how much water lands per unit of area. Move the same heads further apart and the rate drops — same water, spread thinner, so it takes longer.
This is why "run it for 20 minutes" advice is useless. Twenty minutes on a tight 12×12 grid delivers roughly twice what it does on an 18×18 grid, and neither number means anything until you know your spacing and nozzle output.
Head-to-Head Coverage Isn't Optional
Sprinklers throw far more water near the head than at the edge of their arc — the distribution is nothing like even.
The fix is counterintuitive: each head must throw all the way to the next head, so the thin outer edge of one lands on top of the thin outer edge of another. Space them at their full radius, not their full diameter. Skimp here and you get dry rings you'll try to fix by running everything longer, which just floods the areas that were already fine.
Your Supply Decides Your Zones
You cannot run more heads than your water supply feeds. Total demand above your available flow means every head runs at low pressure, the spray pattern collapses, and coverage goes with it.
To find your real number, time how long a 5-gallon bucket takes to fill from an outdoor tap: 30 seconds means 10 GPM. Then divide your heads into zones that each fit inside it, comfortably. Never design to your maximum.
Deep and Infrequent, Always
Daily light watering keeps the surface wet and teaches roots there's no reason to go deeper. You get a shallow root system that panics the moment you skip a day.
Two long soaks a week drives water deep and roots follow it down. The lawn gets more drought-tolerant, not less — and it's the same total water either way. This calculator assumes you're splitting the weekly requirement across your chosen days for exactly that reason.
Water Before Dawn
Early morning loses the least to evaporation and lets the blades dry as the sun comes up. Midday wastes a good fraction to the air before it lands.
Evening is the one to avoid: grass that stays wet all night is how fungal disease starts. The convenient time is the wrong time.
Check It With Tuna Cans
Put a few straight-sided cans around the zone and run it for 15 minutes. Measure what's in each one.
That's your real precipitation rate and your real uniformity, and it beats every calculation on this page — including this one. Any calculator gives you a starting point; the cans tell you what your system actually does.
Related: sod calculator, grass seed calculator, fertilizer calculator.
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