Your city tells you when you can water. Nobody tells you how much. This calculator combines real ET data from your location, measured rainfall from your nearest weather station, and your sprinkler output — and gives you an answer in minutes.
About the math: ET0 is reference evapotranspiration — the industry-standard measure of how much water your turf is losing per day. We pull it from Open-Meteo (FAO-56 Penman-Monteith model, same formula used by NRCS, USDA, and ag consultants). Multiplied by a crop coefficient (Kc) for your grass type, it gives turf demand. We subtract 75% of measured rainfall (effective rain) to get the deficit. Then we divide by your zone's measured precipitation rate to get runtime in minutes.
About the catch-cup test: Place 3–5 identical cups (tuna cans are 3" diameter — the industry standard) evenly across a zone. Run the zone for 10 minutes. Measure the volume each cup collected. Average them, then use our Catch-Cup helper to compute PR. Do this once per zone — the number doesn't change unless you swap nozzles.
Compliance data is a guide, not legal advice. Always confirm your exact watering day and blackout hours with your city's water utility. Our presets are good starting points.
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