How long to run sprinklers: minutes per cycle by soil and head type, with the math behind them

Why "20 minutes three times a week" cannot be right

The run times you read online sound precise and cannot all be right, because two numbers are missing. How many inches per hour your heads apply: a fixed spray puts down about 1.4 in in an hour, a rotor about 0.5 in, so 20 minutes means about 0.47 in from sprays and about 0.17 in from rotors. And how fast your soil takes water in: sand soaks it up instantly but holds little; clay holds a lot but takes it in slowly.

So this article gives a table for three soils and two head types, and the formula behind it, so you can run it for your own system.

Find your soil: hand test, drain test, Web Soil Survey

Hand test: take moist, not wet, soil from 6 to 8 in down and work it between thumb and finger. Sand is gritty and will not hold a shape. Loam forms a ball that breaks when you poke it. Clay is sticky, rolls into a thin shiny ribbon an inch or more long. Silty soils feel floury; treat them like loam in the table.

Drain test to confirm: dig a hole about 12 in across and 6 in deep, fill it, let it drain, fill again and time it. Under 10 minutes points to sand, 10 to 30 to loam, over 30 to clay. Over an hour usually means compaction, common on new construction where equipment drove over the future lawn; core aeration helps more than any schedule. The USDA Web Soil Survey maps the native soil for your address, useful as a cross-check, but builders often strip topsoil and bring in fill, so trust the hole in your own yard.

The table: minutes per run by soil and head type

For a full-sun cool-season lawn on hot summer days, SprinklerMap budgets 5 mm (about 0.2 in) of evapotranspiration per day: 1.38 in net per week, about 1.93 in from the heads with system losses. The soil decides how to spread that out. Sand holds little, so water often and short. Clay holds a lot but absorbs slowly, so water less often in short cycles. The last column is how long a spray zone can run on flat ground before water runs off; rotors apply so slowly that they do not run off on any of the three soils.

Adjust for season, region and grass type

The table is the hot-summer case. Daily evapotranspiration varies a lot across the US: low in cool, humid regions, highest in the desert Southwest. Many states publish it, for example CIMIS in California, and smart controllers pull it automatically. The rule: keep the number of run days and scale the minutes by the ratio of your ET to the 0.2 in in the table. At 0.14 in a day, 21 minutes becomes about 15.

Grass type matters too. Warm-season grasses (Bermuda, zoysia, St. Augustine) use roughly a quarter less than cool-season lawns (bluegrass, tall fescue) on the same day; common University of California guidance uses crop coefficients of 0.6 and 0.8. And the most common mistake we see is running August minutes in late September: nights get cooler, dew lasts longer, and the extra water mostly feeds disease. Use the seasonal adjust percentage on your controller or a weather-based controller.

Cycle and soak: how to water clay

On clay a spray zone runs off after under 8 minutes but needs 28. Cycle and soak splits the 28 minutes into four cycles of 7 minutes with at least 30 minutes between them, so each cycle soaks in before the next one starts. Most controllers have a cycle and soak setting or let you add several start times for the same program.

The other fix is the nozzle: rotors or rotary nozzles at around 0.4 to 0.5 in/hr apply less than even clay absorbs and run in one piece. On a new system on heavy soil, that saves the splitting. SprinklerMap calculates the cycles for each zone from its soil and the nozzles you placed.

The formula for your own system

Minutes per week = weekly need in inches ÷ precipitation rate in in/hr × 60. Weekly need is daily ET times seven, divided by efficiency (about 0.72 for a decent sprinkler system). Measure your own zone precipitation rate with the catch-cup test in /enThe catch-cup test: measure sprinkler uniformity and your real inches per hour, then spread the weekly minutes over the run days in the table and check that no single run is longer than the runoff limit for your soil.

Partial shade needs about 20% less, full shade about 40% less. Plant water needs: /enVegetation Types in SprinklerMap: How ET Coefficients Drive Your Watering Schedule. Slopes have their own limits: /enSlope garden irrigation: designing a system without run-off.

Signs of too little or too much water

• Too little: blades fold in the afternoon, footprints stay visible for minutes, color turns blue-gray, recovery only overnight.
• Too much: moss, spongy turf, fast growth that needs constant mowing, brown instead of white roots when you lift a plug, mushrooms and disease.
• Uneven: green and brown patches in the same zone. Then the problem is distribution, not minutes; a catch-cup test shows where.

Frequently asked questions