How to use the sprinkler run time calculator
- Choose how you know the application rate: you know it in inches per hour, you ran a catch cup test, or you know the zone's flow and area.
- Enter the water you want to apply per week, any rain so far this week and the number of watering days.
- Optionally enter the longest a zone can run before water runs off, to split each day into shorter cycles.
- Read the minutes per watering day, the cycles and the minutes per week.
Formula
Application rate from cups = depth in the cups / test minutes x 60
Application rate from flow = 96.25 x gallons per minute / area in square feet
Minutes per day = (weekly depth - rain) / watering days / application rate x 60
Cycles = round up (minutes per day / longest run)
The 96.25 comes from the unit definition: 1 inch of water over 1 square foot is 144 cubic inches, which is 144 / 231 of a gallon, and 60 minutes in an hour.
A zone that applies 1.2 inches per hour
To apply 1 inch a week over 3 days at 1.2 inches per hour, each day needs 1 / 3 inch, which takes 16.7 minutes, or 50.0 minutes a week. With a 10 minute run limit the day splits into 2 cycles of 8.3 minutes.
A catch cup test of 0.35 inch in 15 minutes is 1.4 inches per hour and 14.3 minutes a day. A zone that uses 6 gallons per minute over 900 square feet applies 0.642 inches per hour, which takes 31.2 minutes a day. If 1.2 inches of rain falls, no watering is needed.
Measuring your own rate
Sprinkler types apply water at very different speeds, so the best number is one you measure. Set out several straight-sided cups across the zone, run it for 15 minutes, and average the depth in the cups. Use that average as the depth in the cup test. Uneven coverage shows up as cups with very different amounts.
If you know the flow in gallons per minute and the area, the flow method gives an average rate. It assumes the water spreads evenly over the area, so real coverage may be less even.
Cycles, targets and local rules
If water pools or runs off before a run ends, enter the time at which that happens as the longest run. The tool then splits the day into several shorter cycles with a pause between them, which lets the soil absorb the water. How long to pause is for you to judge.
The weekly depth is a number you enter. How much a lawn needs depends on the grass, soil, sun and season, and many places set watering days or limits, so check local guidance. For related planning, see the grass seed calculator, the sod calculator and the water bill calculator. The mulch and gravel calculator helps with beds.
Assumptions and limits
- The weekly water target, rain and run limit are numbers you enter. The examples are only samples.
- Water is assumed to be applied evenly over the zone, which real sprinklers do not do exactly.
- Soil, slope, wind, evaporation and local watering rules are not modeled.
- Rain credit is subtracted from the weekly target before dividing across the watering days.
Frequently asked questions
How long should I run my sprinklers?
Divide the water you want per watering day by your zone's application rate and multiply by 60. The tool does that and can split the time into cycles. The weekly target is your choice and may follow local guidance.
How do I find my application rate?
Run a catch cup test and use the average depth and the test time, or use the zone's flow and area. Manufacturers also publish rates for sprinkler types.
What is cycle and soak?
It means splitting a long watering into shorter cycles with a pause between them so water soaks in rather than running off. Enter the longest run before runoff and the tool gives the cycles.
Should rain change my run time?
Yes. Enter the rain so far this week and the tool subtracts it from the weekly target. If rain covers the target, no watering is needed.