Irrigation / Watering Schedule Calculator

Irrigation scheduling calculates how long to run sprinklers to deliver the right amount of water.

Water per session = Weekly need ÷ Sessions per week Run time (minutes) = (Water per session ÷ Sprinkler rate) × 60

Note what is missing from that: area. Sprinkler output is measured as a depth per hour, the same way rainfall is, so a zone covering 200 square feet and one covering 2,000 need the same run time to put down the same inch of water. Area decides how many gallons come out of the tap, not how long the timer runs. That surprises almost everyone the first time.

Unit conversions:

• Imperial: Area in sq ft, water in inches/week, sprinkler rate in inches/hour
• Metric: Area in m², water in mm/week, sprinkler rate in mm/hour
• 1 inch = 25.4 mm

Typical water needs:

Typical sprinkler output rates:

Runoff is the limit nobody schedules around

Soil accepts water at its own pace, and past that point the rest runs off the surface and never reaches a root. Roughly, sandy soil manages about an hour before that happens, loam about half an hour, clay about fifteen minutes.

If your calculated run time is longer than that, the answer is not to shorten it. Split the same total into two or three shorter bursts an hour apart and let each one soak in. Controllers call this cycle-and-soak, and on clay it is the difference between an inch of water in the soil and half an inch in the storm drain.

Measure your real rate rather than trusting the table. Set five straight-sided tins around a zone, run it for 15 minutes, and measure the depth in each with a ruler. Multiply the average by four and you have inches per hour for your system at your pressure, which is frequently half what the catalogue claims.

Tips:

• Water early morning (4–8 AM) to reduce evaporation
• Split into 2–3 sessions per week rather than daily light watering
• Deep, infrequent watering encourages deeper root growth
• Reduce by 30% in spring and autumn, and skip entirely after real rain