Irrigation Runtime Calculator

Landscape preset:

Pick the closest zone first; choose Custom when you want every assumption to stay manual.

System type:

Use Sprinkler for in/hr or nozzle spacing; use Drip for emitter count, dripline, or GPH flow.

Water need basis:

Choose ETo for peak-week planning; choose Target depth when the event depth is already set.

Schedule mode:

Manual uses 1-7 days/week; soil reserve is available only with the ETo demand basis.

Peak-week ETo:

Enter local reference ETo as mm/day, mm/week, in/day, or in/week.

Crop coefficient:

Typical Kc: turf 0.60-0.85, shrubs 0.40-0.70, vegetables 0.90-1.05.

Target depth per event:

Enter net depth per start as millimetres or inches.

Watering days per week:

Enter an integer from 1 to 7 that matches the watering days allowed this week.

Sprinkler rate source:

Measured is best after an audit; derived needs nozzle flow, arc, head spacing, and row spacing.

Precipitation rate:

Use a positive in/hr or mm/hr rate; sprays often 1.2-2.0 in/hr, rotors often lower.

Nozzle flow:

Enter positive flow for one head, using GPM or L/min from the nozzle chart or test.

Arc coverage:

Use 90 for quarter-circle, 180 for half-circle, or 360 for full-circle heads.

deg

Head spacing × row spacing:

Enter positive spacing values, such as 15 x 15 ft for square head-to-head coverage.

Water need basis:

Choose ETo when area is known; choose Weekly plant demand when you already know litres/week or gallons/week.

Schedule mode:

Manual uses 1-7 days/week; soil reserve is available only with ETo + area demand.

Peak-week ETo:

Enter local reference ETo as mm/day, mm/week, in/day, or in/week.

Crop coefficient:

Use plant demand, not emitter style: shrubs often 0.40-0.70, vegetables 0.90-1.05.

Weekly plant demand:

Enter net plant demand before efficiency and uniformity losses as litres/week or gallons/week.

Watering days per week:

Enter an integer from 1 to 7; fewer days make each drip start longer.

Drip layout:

Use point-source for a known count; use inline dripline when spacing and bed area define emitters.

Emitter flow:

Enter flow per emitter in GPH or LPH; common residential emitters are 0.5-2.0 GPH.

Emitter count:

Enter a whole number of active emitters, minimum 1.

Emitter spacing:

Enter positive spacing along the tubing, such as 12 in or 30 cm.

Lateral spacing:

Enter positive spacing between dripline rows, such as 18 in or 45 cm.

Irrigated area:

Enter the active zone area in square feet or square meters; required for ET drip and inline dripline.

Soil profile:

Select the closest texture; changing it also refreshes the default infiltration rate.

Root depth:

Enter positive active root depth; use conservative values for new turf or shallow annuals.

Allowable depletion: {{ mad_percent }}%

Range: 15-80%; lower values create more frequent, lighter irrigation starts.

Irrigated area:

Optional for sprinklers; enter square feet or square meters to show gallons per event.

Effective rainfall:

Enter usable rain as in/week or mm/week; use 0 when rainfall should not offset irrigation.

Application efficiency:

Enter 1-100%; defaults often start near 75% for spray and 90% for drip.

Uniformity (DU):

Enter 1-100%; use catch-can DU for sprinklers or audit estimates for drip zones.

Root depth:

Enter positive active root depth; use conservative values for new turf or shallow annuals.

Allowable depletion: {{ mad_percent }}%

Range: 15-80%; lower values create more frequent, lighter irrigation starts.

Infiltration rate:

Enter positive in/hr or mm/hr; lower values make cycle-soak splitting more likely.

Soak between cycles:

Enter minutes of pause between cycles; use 0 only when no soak break is planned.

min

Section	Item	Adjustment	Runtime	Detail
Controller line	{{ planHeadline }}	Peak week	{{ planSubline }}	Baseline controller recommendation.
Controller detail	{{ row.label }}	-	{{ row.value }}	Current input translated for controller setup.
Adjustment ladder	{{ row.label }}	{{ row.percent }}%	{{ row.eventRuntime }} / {{ row.weeklyRuntime }}	Starting point relative to the peak-week runtime.
Runtime recommendation	{{ item.title }}	{{ item.badge }}	-	{{ item.detail }}

Metric	Value	Copy
{{ row.label }}	{{ row.value }}

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Landscape irrigation time is the visible setting on a controller, but it is only the last step in a water budget. A zone may need ten minutes, forty minutes, or several short cycles because of plant demand, sprinkler rate, drip flow, soil storage, and how much water is lost before it reaches the active roots. Changing any one of those pieces can move the runtime more than changing the controller by a minute or two.

Most practical schedules begin with a target amount of water. In weather-based scheduling, reference evapotranspiration, usually written as ETo, estimates how much water a reference surface would lose to evaporation and plant transpiration. A crop coefficient, Kc, adjusts that reference value for turf, shrubs, vegetables, trees, or another planting type. Effective rainfall is subtracted only when the rain is likely to reach the root zone and replace irrigation water. In direct scheduling, the target is already known as a depth for sprinklers or a weekly volume for drip.

Runtime also depends on the delivery system. Sprinklers are usually described by precipitation rate, such as inches per hour or millimetres per hour. Drip zones are usually described by emitter flow and emitter count, such as gallons per hour or litres per hour. A sprinkler with a high precipitation rate can finish quickly but may outrun the soil's intake rate. A drip zone with low-flow emitters can run for a long time and still apply a modest total volume.

Several terms matter because they explain why two similar yards can need different schedules. Distribution uniformity describes how evenly water reaches the zone, so a low value means some dry areas need longer gross runtime. Application efficiency accounts for losses such as wind drift, overspray, evaporation, or water that misses the intended root area. Allowable depletion is the share of stored soil water that can be used before irrigation should refill the root zone.

A common mistake is to treat the calculated minutes as proof that the landscape is receiving water evenly. The runtime may be mathematically consistent while pressure is low, heads are tilted, emitters are plugged, slopes are shedding water, or one plant group has a much shallower root zone than assumed. Another mistake is mixing time scales, such as using daily ETo as though it were a weekly value, or subtracting all rainfall even when much of it ran off or never reached the irrigated bed.

A sound irrigation runtime is therefore a planning number, not a replacement for observation. It gives a controller starting point that can be adjusted after catch-can tests, emitter checks, soil probes, seasonal demand changes, or hydrozone separation show that one valve is serving more than one watering need.

How to Use This Tool:

Start with the closest landscape preset so the calculator can seed realistic assumptions for plant demand, soil profile, root depth, efficiency, and common system type. Choose Custom zone when you already have audit values and do not want preset assumptions to steer the setup.

Select System type. Use Sprinkler when the zone is controlled by depth and precipitation rate. Use Drip when runtime depends on emitter flow and total zone volume.
Pick the Water need basis. Sprinklers can use ETo + crop coefficient or a fixed Target depth per event. Drip can use ETo + crop coefficient + area or a known Weekly plant demand.
Set Schedule mode. Manual watering days uses the number of starts allowed this week. Soil reserve interval is available for ETo-based demand and estimates frequency from root depth, soil water holding capacity, and allowable depletion.
Enter the delivery fields for the selected system. For sprinklers, use a measured precipitation rate when you have a catch-can result, or derive a rate from nozzle flow, arc coverage, head spacing, and row spacing. For drip, enter emitter flow and either a counted emitter total or inline dripline spacing with irrigated area.
Review the soil and advanced assumptions. Effective rainfall, application efficiency, distribution uniformity, infiltration rate, root depth, allowable depletion, and soak minutes all affect the schedule or the cycle-soak warning.
Read Controller Plan first, then check Water Ledger, Water Budget Map, Cycle/Soak Map, and JSON when you need audit details, chart exports, or a machine-readable record.

Fix any validation message before trusting the result. Common blockers include zero precipitation rate, zero emitter flow, missing drip area, invalid inline spacing, and root depth values that leave no soil reservoir to model.

Interpreting Results:

The headline schedule is a peak-week controller recommendation. It reports runtime per event and the effective number of watering starts per week. If ETo demand is fully offset by effective rainfall or the demand inputs produce zero water need, the summary can tell you to pause the zone for the design week.

Run time / event is the main minutes-per-start value for the controller.
Run time / week helps compare seasonal adjustments and confirms how much total time the valve will run across the week.
Gross water / event includes application efficiency and distribution uniformity losses, so it should be greater than net plant demand whenever those percentages are below 100%.
Root-zone reserve and Soil interval ceiling estimate frequency from stored soil water. They do not prove that every dry spot receives the same water.
Cycle plan appears when the application rate is faster than the selected infiltration rate. Use the split cycles and soak gap instead of one continuous run.

The recommendation rows call out practical checks such as a schedule that is lighter than the soil model, frequent starts that may keep the surface too wet, low efficiency that stretches runtime, very short sprinkler sets that deserve catch-can verification, or long drip sets that may be normal for low-flow zones.

Treat the charts as comparison aids. Water Budget Map compares net event water, gross event water, weekly gross water, and soil reserve when available. Cycle/Soak Map shows the running and soaking sequence for one watering day, which is useful when a controller needs multiple starts and pause times.

Technical Details:

Irrigation runtime combines a demand calculation with a delivery-rate calculation. Demand is first expressed as a net depth or net volume. Losses then inflate that amount into a gross application target. The final runtime divides the gross target by how fast the zone applies water.

Depth and volume are interchangeable only after area is known. One inch of water over one square foot is about 0.623 gallons, so an ETo depth can become a drip volume only when the irrigated area is supplied. Sprinkler runtime can be calculated from depth alone because precipitation rate is already a depth per hour.

Formula Core:

The calculation uses separate runtime equations for sprinklers and drip, but the same loss factor and water-budget idea applies to both paths.

\begin{array}{lcl} D_{week} & = & \max ({ETo}_{week} \times Kc - R_{effective}, 0) \\ L & = & Efficiency \times DU \\ T_{sprinkler} & = & \frac{D_{event} / L}{PR} \times 60 \\ T_{drip} & = & \frac{V_{event} / L}{Q_{zone}} \times 60 \end{array}

In these equations, Dweek is weekly net water depth, Devent is net depth per sprinkler event, Vevent is net drip volume per event, PR is sprinkler precipitation rate in inches per hour, and Qzone is drip zone flow in gallons per hour. Efficiency and DU are entered as percentages but used as fractions.

Irrigation runtime calculation paths
Path	Demand source	Delivery source	Runtime result
Sprinkler, ETo	Weekly ETo multiplied by Kc, reduced by effective rainfall, then split across starts.	Measured precipitation rate or nozzle flow derived from arc and spacing.	Gross event depth divided by precipitation rate.
Sprinkler, target depth	Typed net depth for one controller start.	Measured or derived precipitation rate.	Gross target depth divided by precipitation rate.
Drip, ETo	Weekly ETo demand converted from depth to gallons using irrigated area.	Emitter flow multiplied by counted or inferred emitters.	Gross event volume divided by zone flow.
Drip, weekly demand	Typed net gallons or litres per week, split across watering days.	Emitter flow multiplied by counted or inferred emitters.	Gross event volume divided by zone flow.

Derived sprinkler precipitation rate uses the individual-head form of the standard precipitation equation:

PR = \frac{34650 \times GPM}{Arc \times HeadSpacing \times RowSpacing}

For dripline, the emitter count is rounded up from area divided by the coverage rectangle formed by emitter spacing and lateral spacing. Drip application rate is then calculated as zone gallons per hour times 1.604 divided by square feet, giving an approximate inches-per-hour rate for comparing drip output with soil intake.

Irrigation runtime constants and boundaries
Item	Rule used	Why it matters
Unit conversion	25.4 mm per inch, 3.785 litres per gallon, 10.764 square feet per square metre.	Keeps mixed metric and imperial inputs on one calculation base.
Depth to volume	Gallons = inches x square feet x 0.623.	Turns ETo depth into drip demand when area is known.
Efficiency and DU	Application efficiency and distribution uniformity are each used as 1% to 100% fractions, then multiplied.	Models gross water needed when losses or uneven delivery stretch runtime.
Soil reserve	Available water holding capacity x root depth x allowable depletion, with depletion in a 15% to 80% planning range.	Estimates how long the root zone can dry down before refill.
Soil interval	Reserve divided by daily net demand, with watering days rounded to a 1 to 7 day/week schedule.	Converts soil storage into controller frequency.
Cycle-soak trigger	Application rate greater than infiltration rate.	Splits a long run when water is being applied faster than the soil intake setting.
Cycle length	When splitting is needed, maximum cycle minutes use 85% of the intake-to-application ratio.	Adds a conservative margin before estimating the number of cycles.

A sample sprinkler calculation shows the scale of the adjustments. With 5.6 mm/day ETo, Kc 0.80, no effective rain, and three watering days per week, net demand is about 31.4 mm/week and net depth per start is about 10.5 mm. With 75% efficiency and 75% DU, the gross depth becomes about 18.6 mm per start. At a 35 mm/hr precipitation rate, runtime is about 31.9 minutes before any cycle-soak split is considered.

Rounding is mainly for display. The underlying calculation keeps fractional values, then output tables and charts format minutes, depths, flow rates, and volumes to readable precision. A controller still may need whole-minute programming, so round final minutes in the direction that fits the landscape risk: down when runoff or wet soil is the concern, up when dry stress is the concern and field checks support it.

Limitations and Privacy:

The calculator does not measure pressure, catch-can distribution, emitter clogging, slope runoff, wind drift, water restrictions, salinity stress, plant disease risk, or actual soil moisture. It also does not know whether different plants on the same valve should be separated into different hydrozones. Use the result as a controller starting point and revise it after field observations.

Calculations happen in the browser for the values entered on the page. Copy, download, chart image, DOCX, CSV, and JSON actions create outputs only when selected, so avoid exporting sensitive site notes if the file will be shared outside your project.

Worked Examples:

A turf sprinkler zone might use ETo demand with three watering days per week, measured precipitation rate, loam soil, and a moderate loss factor. If the gross event depth divided by precipitation rate produces a 32 minute runtime but the application rate exceeds the infiltration setting, the controller plan may split the start into two shorter cycles with a soak gap.

A shrub drip zone with a known weekly plant demand can skip the ETo and area demand path. For example, 80 litres per week across three watering days is about 26.7 litres net per event. With 90% efficiency, 90% DU, and 24 emitters at 3.8 L/hr each, the gross event volume is about 32.9 litres and the runtime is about 22 minutes per start.

An inline dripline bed needs area, emitter spacing, lateral spacing, and emitter flow. If the spacing values imply more emitters than were actually installed, the calculated zone flow will be too high and runtime will be too short. Count or test the active emitters when the result looks unexpectedly small.

FAQ:

What is the difference between ETo and Kc?

ETo is weather demand for a reference surface. Kc adjusts that reference value for the plant type and growth condition, so ETo x Kc estimates plant water use for the chosen landscape.

Why is drip runtime sometimes much longer than sprinkler runtime?

Drip emitters apply water at low flow rates. A long drip runtime can still represent a modest total volume, especially when the zone has few emitters or low L/hr or GPH output.

Why did cycle-soak appear?

Cycle-soak appears when the selected application rate is higher than the soil infiltration setting. Splitting the runtime gives water time to enter the soil before the next run begins.

Should I use measured precipitation rate or nozzle flow and spacing?

Use measured precipitation rate when you have catch-can data for the zone. Use nozzle flow and spacing when planning from product charts or when a measured rate is not available.

Why does gross water exceed net demand?

Gross water includes losses from application efficiency and distribution uniformity. Lower percentages require more applied water so the driest parts of the zone receive the intended net amount.

Glossary:

ETo: Reference evapotranspiration, the weather-based demand value used before plant-specific adjustment.
Kc: Crop coefficient, the multiplier that adjusts reference ETo to a plant or landscape type.
Distribution uniformity: A percentage describing how evenly water reaches the zone. Lower uniformity increases gross runtime.
Application efficiency: The share of applied water expected to reach the intended root zone after practical losses.
Allowable depletion: The portion of stored root-zone water allowed to be used before irrigation should refill the soil.
Cycle-soak: A controller pattern that divides one irrigation event into shorter runs separated by soak time.

References:

FAO Irrigation and Drainage Paper 56, Chapter 6: ETc - Single Crop Coefficient, Food and Agriculture Organization of the United Nations.
Evapotranspiration-based irrigation scheduling or water-balance method, University of Minnesota Extension.
What is acceptable distribution uniformity (DU)?, University of California Agriculture and Natural Resources.
Weather-Based Irrigation Controllers, U.S. Environmental Protection Agency WaterSense.
Irrigation water: How it's delivered, how it's measured, Oregon State University Extension Service.