How long should an irrigation set run?
A "set" is one position of a surface or sprinkler irrigation system that's watered before the crew, the machine or the valve controller moves on to the next one: one lateral of hand-move or solid-set pipe, one stop along a wheel line, or one furrow or flood block. Every set needs to run long enough to deliver a target depth of water over its area, and that time depends on only three things: how big the set is, how deep you want the water, and how fast water is arriving.
Those three combine into a straightforward volume relationship. The water you need is area × depth; the water you're getting is flow rate × time. Setting those equal and solving for time is what this calculator does, once the units are converted to match.
| Net application | 70% efficiency | 80% efficiency |
|---|---|---|
| 0.10 in/day | 2.69 gpm/acre | 2.36 gpm/acre |
| 0.20 in/day | 5.39 gpm/acre | 4.71 gpm/acre |
| 0.30 in/day | 8.08 gpm/acre | 7.07 gpm/acre |
| 0.40 in/day | 10.78 gpm/acre | 9.43 gpm/acre |
| 0.50 in/day | 13.47 gpm/acre | 11.79 gpm/acre |
These figures come from Kansas State University Research and Extension's published delivery-rate table. If your flow rate divided by your set's acreage falls far outside this kind of range for the daily rate you're aiming for, double-check your flow measurement or your target depth before trusting the set time below.
Well and canal capacity limits apply first. If your water right, well permit or district allocation caps your flow rate or seasonal volume, that cap governs regardless of what a calculated set time suggests.
How this irrigation set time calculator works
The calculator turns your target depth and set size into a volume of water, then divides by your flow rate to get hours. If you add your daily operating window and total field size, it also works out how many sets fit in a day and how many days a full cycle across the field takes.
1. Hours for one set
US units
Hours = 453 × Set size (acres) × Target depth (in) ÷ Flow rate (GPM)
453 is the rounded gallons per minute it takes to deliver one acre-inch of water in an hour. The exact figure is about 452.6: one acre-inch is 27,154 gallons, and 27,154 ÷ 60 minutes ≈ 452.6. 453 is the simpler constant used here, matching the rounding common to published irrigation-engineering calculators.
Metric units
Hours = (10,000 × Set size (ha) × Target depth (mm)) ÷ (3,600 × Flow rate (L/s))
One millimeter of water over one square meter is exactly one liter, so one hectare (10,000 m²) needs 10,000 liters per millimeter of depth applied. Dividing by 3,600 seconds per hour converts the flow rate into liters per hour.
2. Sets per day
Sets per day
Sets per day = Hours available per day ÷ Hours per set, rounded down to a whole set
Rounded down because a set that isn't finished doesn't count as complete; it carries into the next day instead.
3. Sets and days to cover the field
Covering the whole field
Sets needed = Total field size ÷ Set size, rounded up
Days for one cycle = (Hours per set × Sets needed) ÷ Hours available per day, rounded up
Days are calculated from total hours needed, not from the sets-per-day figure alone, so the result stays correct even when a single set already runs longer than one day's window.
Useful conversions
| Quantity | Value |
|---|---|
| 1 acre-inch | 27,154 gallons |
| 1 acre-inch per hour | about 452.6 GPM |
| 1 mm of water over 1 m² | 1 liter |
| 1 acre | 0.4047 hectare |
| 1 GPM | 0.0631 L/s (3.785 L/min) |
Hours per set is shown to the nearest minute. Sets needed and sets per day are always whole numbers: rounded up when a partial set still means another full pass is required, and rounded down when only complete sets count toward a day's total. For a quick depth-to-volume conversion on its own, use the acre-inches calculator.
When this calculator isn't the right tool
It's built for irrigation systems that sit at one position for a set period before moving on. Use a different approach for:
- Choosing the depth to apply in the first place, rather than the time to apply it. See the irrigation application depth calculator, which uses the same relationship solved for depth instead of time.
- Center pivots and travelers, which move continuously while applying water rather than sitting at fixed sets. See the center pivot calculator or the traveler irrigation calculator.
- Drip and micro-irrigation, where run time depends on emitter output rather than a lateral's total flow. See the drip run time calculator.
- Residential lawn sprinkler zones. See the sprinkler run time calculator or the lawn watering calculator.
- Sizing the well or pump itself, rather than scheduling sets from a known flow rate. See the irrigation well capacity calculator.
Irrigation set time calculation example
A grower runs a solid-set lateral covering 20 acres, targets 1.5 inches of water for this irrigation, and measures 700 GPM feeding that lateral. They can run the system continuously (24 hours a day), and the whole field this lateral has to cover totals 100 acres.
- Hours per set: 453 × 20 × 1.5 ÷ 700 = 19.414 hours, shown as about 19 hr 25 min.
- Sets per day: 24 ÷ 19.414 = 1.24, rounded down to 1 set per day.
- Sets needed to cover the field: 100 ÷ 20 = 5 sets.
- Days for one full cycle: (19.414 × 5) ÷ 24 = 4.04, rounded up to about 5 days.
The same calculation in metric
A 5-hectare set with a 25 mm target, fed at 20 L/s, run up to 24 hours a day, across a 20-hectare field:
- Hours per set: (10,000 × 5 × 25) ÷ (3,600 × 20) = 17.361 hours, shown as about 17 hr 22 min.
- Sets per day: 24 ÷ 17.361 = 1.38, rounded down to 1 set per day.
- Sets needed to cover the field: 20 ÷ 5 = 4 sets.
- Days for one full cycle: (17.361 × 4) ÷ 24 = 2.89, rounded up to about 3 days.
Notice that a single set can already run more than a day's operating window without breaking the field-level result: the days-to-finish figure is calculated from total hours needed, not from how many whole sets fit in one day.
Practical guidance for scheduling set times
Measuring your flow rate
The flow rate you enter should be what's actually reaching this one set, not your pump's total rated capacity split evenly on paper. If several sets or zones share a mainline, opening or closing others changes the pressure and flow each one gets. A bucket test at an open outlet, timed with a stopwatch, is a fast way to check the real number under actual operating conditions.
Choosing a depth that won't run off
Entering a bigger target depth always produces a longer set time, but the soil has to be able to absorb it. If the depth you want to apply, divided by the hours you're running, exceeds your soil's basic infiltration rate, water will pond and run off before the set finishes. The soil infiltration rate chart gives typical ranges by texture, and the irrigation application depth calculator can help you settle on a depth from your soil's water-holding capacity before you calculate a time for it here.
Common mistakes
- Using nameplate or design flow instead of a measured one. Pumps rarely deliver their rated GPM once real pipe friction, elevation change and valve losses are in the picture.
- Ignoring pressure loss along a long lateral. The far end of a lateral can run noticeably slower than the inlet; check pressure at both ends if the lateral is long or if coverage looks uneven.
- Skipping soak time on slow soils. On clay or compacted ground, splitting one long set into shorter cycles with a break between them lets water infiltrate instead of running off. The cycle and soak calculator works out that split.
- Forgetting seasonal changes in well or canal output. A flow rate measured in spring may not hold through peak summer demand.
Factors that change the set time result
The calculator only knows the numbers you give it. These are the main reasons your real flow, and therefore your real set time, can differ from what you enter.
| Factor | Effect | What to check |
|---|---|---|
| Pressure loss along the lateral | Outlets farther from the inlet can receive less flow than those near it. | Measure pressure or flow at both ends of a long lateral. |
| Seasonal well or canal decline | Static water levels and district demand both shift flow capacity through the season. | Re-measure flow rate periodically rather than using one reading all year. |
| Soil infiltration rate | Applying water faster than the soil absorbs it causes runoff, regardless of the calculated time. | Compare your depth ÷ hours to a sourced infiltration rate for your soil. |
| System application efficiency | Wind drift, evaporation and deep percolation mean less than the gross depth reaches the root zone. | Add a margin to the target depth to cover expected losses. |
| Shared mainline demand | Flow to one set can drop when other valves or zones open at the same time. | Measure flow under the same operating conditions you'll actually run. |
| Wind | Strong wind pushes sprinkler spray off target and increases evaporation loss. | Where possible, schedule sets for calmer, cooler hours. |
Frequently asked questions
What is an irrigation set, and how long should it run?
A set is one position of an irrigation system — one lateral of solid-set or hand-move pipe, one stop on a wheel line, or one furrow or flood block — that's watered before moving on to the next. How long it should run depends on the depth of water you want to apply, the size of that set, and the flow rate feeding it: more depth or more area take longer, and more flow shortens the time.
How do I find the flow rate for one set?
Measure it directly if you can, with a flow meter on the mainline or a bucket test at an open outlet. If you can't measure it, use your pump's rated output from its performance curve at the pressure your system is actually running, divided among however many outlets are open at once. A flow rate that's assumed rather than measured is one of the most common reasons a calculated set time turns out wrong.
How much depth should I apply per set?
That depends on your soil's available water capacity and how far it's been depleted since the last irrigation, not on one fixed number. Coarser, sandier soils hold less water and need lighter, more frequent sets; finer soils can take a deeper set less often. Applying more than your soil's basic infiltration rate in the time you're running causes runoff, so check that rate too before entering a depth here.
See the soil infiltration rate chart or the irrigation application depth calculator.
Why does my measured flow rate change through the season?
Well and canal flows can drop in mid-summer as static water levels fall or demand rises across a district, and a pump's output also depends on the operating pressure, which shifts with elevation change and how many outlets are open at once. Re-check your flow rate periodically rather than assuming the value you measured once still holds.
What if the calculated set time is longer than I can supervise?
Split the target depth across two shorter sets, use automated valves that don't need an operator present, or accept a lighter single application and irrigate more often. This calculator's sets-per-day result flags when a single set already runs past the daily window you enter.
Does this calculator account for system efficiency or losses?
No. It calculates the gross time to deliver the depth you enter at the flow rate you enter, assuming all of that water lands where you intend. Wind drift, evaporation, deep percolation below the root zone and uneven distribution all reduce how much actually benefits the crop, so many growers add a margin to the target depth to allow for those losses before entering it here.
Can I use this for a center pivot or a drip system?
Not directly. Center pivots apply water continuously while rotating rather than in fixed sets, with their own speed-and-depth relationship. Drip and micro-sprinkler systems are also usually run to a target depth, but at much lower flow rates and often for many hours at a time.
See the center pivot calculator or the drip run time calculator.
How is sets per day different from hours per set?
Hours per set is how long one position needs to run to deliver your target depth. Sets per day is how many of those complete sets fit inside the hours you're actually able to operate — for example, three 6-hour sets fit into an 18-hour window, but only two fit into 16 hours.
Who built and checked this calculator
Licensed Professional Engineer and Irrigation Association Certified Irrigation Designer with more than 20 years in irrigation design, system auditing and landscape water management. Full profile
How this page was checked
- Formula, unit conversions and both worked examples verified by hand against the source conversions below.
- Guidance checked against the extension sources below.
- The set-time and delivery-rate relationships follow standard irrigation-engineering unit conversions (acre-inches, GPM, and the area × depth = flow × time relationship).
Sources
- Kansas State University Research and Extension. Rogers, D.H. and Alam, M. Irrigation Formulas and Conversions (MF2404).
- Colorado State University Extension and USDA Natural Resources Conservation Service. Bauder, T., Schneekloth, J. and Wardle, E. Irrigated Field Guide & Record Book (XCM-228).
- Oregon State University Extension Service. Irrigation Water Scheduling (EM 9717).
- University of Arizona Cooperative Extension. Martin, E.C. Determining the Amount of Irrigation Water Applied to a Field (AZ1157, Arizona Water Series No. 29). [VERIFY URL]