Sprinkler Run Time Calculator
How long to run each zone for an inch of water — and why a spray head on clay wastes most of it after six minutes.
How fast each head applies water
An inch a week is right. 34 minutes of it isn't.
A fixed spray head applies about 1.75 inches an hour. Clay soil accepts about 0.15. The sprinkler is delivering water nearly twelve times faster than the ground can take it, so after about six minutes the surface has filled and everything after that runs into the street. The weekly total isn't the hard part — delivering it is, and a controller set to the right total can still put most of it down the drive.
Head type changes the run time threefold. A spray puts down 1.75 in/hr and a rotor about 0.5, so the same inch takes 34 minutes from one and two hours from the other. Which is why you can't mix head types on a zone: whatever time you set, half the lawn is wrong, and no amount of adjusting the controller fixes a plumbing decision.
The way out is cycle and soak. Run until just before runoff, stop, let it infiltrate, run again. Same total water, delivered in portions the soil can absorb — and it costs nothing but controller programming. It's the single biggest improvement available to most systems.
- Measure, don't assume. Put half a dozen straight-sided cans out, run the zone 15 minutes and multiply by four. Real zones commonly differ from the catalogue by a factor of two — and vary nearly as much across their own coverage, which is what makes one corner brown.
- Deep and infrequent beats little and often. The same weekly total split over seven days keeps roots in the top inch of soil, where they die the first week nobody waters.
- Slope makes everything worse. It cuts both the infiltration rate and the surface storage, so a steep zone runs off far sooner than a flat one with identical soil.
- Water between 4 and 9 am. Least wind, least evaporation, and the leaf dries as the sun comes up — evening watering leaves it wet all night, which is what fungal disease wants.
- Rain counts in full. A controller that doesn't know it has rained is the commonest source of overwatering there is.
How to use
- Pick the head type on the zone — it changes the run time threefold.
- Pick the soil, since that decides how fast the water can go in.
- Enter the weekly water wanted and any rain that has already fallen.
- Run the cycle-and-soak schedule if the tool says the zone runs off.
Frequently asked questions
How long should I run my sprinklers?
For an inch of water, about 34 minutes from a fixed spray head, two hours from a rotor, and two and a half from a rotary nozzle. There is no single answer because the run time is just the depth divided by the precipitation rate, and the rate depends entirely on what is screwed into the riser. A controller set from a magazine article is set for somebody else’s sprinklers.
Why does my lawn water run into the street?
Because the sprinkler applies water faster than the ground can take it. A fixed spray puts down about 1.75 inches an hour and clay soil accepts about 0.15 — nearly twelve times slower. The surface holds a little, roughly a seventh of an inch on flat ground, and once that fills everything after it runs off. On that combination it takes about six minutes.
What is cycle and soak?
Splitting a run into shorter cycles with a soak between them, so each cycle stops before runoff starts and the water has time to infiltrate. The total water is identical; only the delivery changes. It costs nothing but controller programming and it is the single biggest improvement available to most systems — a zone that was losing half its water down the drive now puts all of it in the ground.
What is a precipitation rate?
Inches of water applied per hour, which is the only number that converts a watering target into a run time. Fixed sprays are around 1.75, rotors around 0.5, rotary nozzles and impacts around 0.4, and hose-end oscillating sprinklers somewhere near 0.7 depending on pressure. Everything else in irrigation scheduling follows from it.
Can I mix spray heads and rotors on the same zone?
No, and this is the reason more than any other that lawns have permanently wet and permanently dry patches. The same inch takes 34 minutes from a spray and 120 from a rotor, so whatever time the controller is set to, one half of the zone gets roughly three times what the other does. It is a plumbing mistake and no amount of controller adjustment fixes it.
How do I measure my own precipitation rate?
Put half a dozen straight-sided containers — tuna cans work — around the zone, run it for fifteen minutes, then measure the depth in each and multiply by four to get inches per hour. Real zones commonly differ from the catalogue figure by a factor of two, and the variation between cans in the same zone is nearly as large, which is what makes one corner brown while the rest looks fine.
Is an inch of water a week the right target?
It is a reasonable starting point for cool-season turf in mild weather, and it can be half that in spring or double in a heatwave. The lawn tells you sooner than the calendar does: grass that stays flattened after being walked on, or that takes on a blue-grey cast, is asking for water. The weekly figure is not the hard part — delivering it without runoff is.
Should I water a little every day?
No. Daily light watering keeps the roots in the top inch of soil, where they die the first week nobody waters. The same weekly total delivered once or twice drives roots down to where the moisture is, which is what makes a lawn survive being forgotten. Deep and infrequent is genuinely better for the grass, not merely cheaper.
What time of day should I water?
Between about four and nine in the morning. The wind is lowest, evaporation is least, and the leaf dries as the sun comes up. Evening watering leaves the grass wet all night, which is what fungal disease wants, and midday watering loses a real fraction to evaporation before it reaches the soil at all.
Does slope change how long I can run a zone?
Considerably, and in both terms at once. A slope reduces the effective infiltration rate and also reduces how much water the surface can hold before it starts moving downhill, so a steep zone can run off in under half the time of a flat one with identical soil. Slopes almost always need cycle and soak even where the flat parts of the same lawn do not.
Does rainfall count towards the weekly total?
In full. An inch of rain is an inch of water and replaces an inch of irrigation. A controller that does not know it has rained is the commonest source of overwatering there is, which is why a rain sensor — a very cheap device — pays for itself in a season and is required by code in a good many places.
Why are rotary nozzles recommended for clay?
Because their whole purpose is the low application rate. They retrofit into an existing spray body and drop the rate from about 1.75 inches an hour to about 0.4, which turns a zone that runs off in six minutes into one that can absorb what it is given. The run time roughly quadruples, which is the trade, but the water goes into the ground rather than down the drive.
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