How this calculator works
The published formula is the formula that runs. There is no hidden coefficient and no lookup table anywhere in this tool.
net = max(0, weekly depth − rain) inches
zone net = net ÷ 2 if two zones cover the same ground (CSU)
= net otherwise
per day = zone net ÷ watering days inches
rate = measured catch-can depth in/hr
OR 1.75 spray / 0.5 rotor (CSU working estimates)
run time = (per day ÷ rate) × 60 minutes
cycles = ceil(run time ÷ runoff minute) if runoff observed
and run time > runoff
= 1 otherwise
per cycle = run time ÷ cycles
soak = at least 60 minutes between start times (CSU)Worked, at the defaults. One inch a week, no rain, two watering days, pop-up spray heads: net = 1 in, per day = 0.5 in, rate = 1.75 in/hr, run time = (0.5 ÷ 1.75) × 60 = 17 minutes, twice a week. Switch the head type to rotors at 0.5 in/hr and the same half inch needs (0.5 ÷ 0.5) × 60 = 60 minutes.
Now add an observation: you watch the run and water starts moving onto the path at 8 minutes, and your measured rate is 1.4 in/hr. Run time = (0.5 ÷ 1.4) × 60 = 21 minutes. 21 ÷ 8 = 2.6, rounded up to 3 cycles of 7 minutes, start times an hour apart. Without the split, roughly 0.3 inches would have been applied to ground that had already stopped accepting it.
What each input means
| Input | What it is actually asking |
|---|---|
| Weekly water depth | How much water the lawn needs in a week, including rain. EPA WaterSense’s starting point is one inch and it explicitly says this varies by location, recent weather and planting. WHERE TO FIND A BETTER ONE: many water districts and conservation programmes publish a weekly watering number for their own service area, updated through the season — search your district’s name plus "weekly watering". That figure beats any national default, ours included. |
| Rain this week | Rainfall already received, in inches, which is subtracted because EPA’s target is inclusive of it. A cheap rain gauge sited in the open is the honest source; a phone weather app reports the nearest station, which can be miles away and a different shower. |
| Watering days per week | How many days you intend to run the system. CSU’s guidance for Colorado soils is once a week in spring and autumn, twice a week in summer, with the note that watering as infrequently and as deeply as the soil allows gives better resilience in hot spells and suppresses some weeds. |
| Precipitation rate — measured or estimated | Choose measured only if you have actually run the test. The tool changes its own caveats based on this answer, and tells you plainly when the minutes rest on an estimate rather than a measurement. |
| Measured rate, inches per hour | The depth of water in the combined can after the six-can, ten-minute test described in step 4 above. Measure with a ruler. Record it per zone; it does not change until the hardware does. |
| Head type | Used only when you have not measured. Pop-up spray heads are the short fixed fan-shaped sprays; rotors are the ones that sweep back and forth in a stream. CSU publishes 1–2½ in/hr and ¼–¾ in/hr respectively, and we use their working estimates of 1.75 and 0.5. These are starting points and CSU says so. |
| Minutes until pooling or runoff | Stand outside during one run with a phone timer and note when water first pools on the surface or moves onto a path or drive. If it never happens, enter 0 and no cycles are applied. This one observation replaces the soil-infiltration table other calculators use — and it is a measurement of your lawn rather than an average of a soil class. |
| Overlapping zones | Whether the patch you are scheduling is watered by two zones from opposite sides, which is common in narrow strips and around corners. If so CSU’s rule is to halve each zone’s run so that together they apply the intended depth. Look at where the spray from each zone actually lands. |
Worked examples
Including one where the naive answer misleads, which is the example most calculators leave out.
The default: spray heads, one inch a week, two days
No rain, two watering days, pop-up spray heads, no runoff observed. The tool returns 17 minutes per watering day, on 2 days a week, as one continuous run. The arithmetic line reads: 1 in to apply over 2 days — 0.5 in per watering day at 1.75 in/hr, based on the CSU working estimate. And the limits line does not let it pass unqualified: you used a head-type estimate, which CSU publishes as a starting point and explicitly says does not account for the actual application rate of your zones. Run the catch-can test before trusting the minutes.
Rotors, same water, three and a half times the clock
Change nothing but the head type. The tool returns 60 minutes per watering day for the identical half inch. This is the single most useful comparison on the page: anyone who copies a neighbour’s "twenty minutes" across a fence between a spray system and a rotor system is out by a factor of three or four in one direction or the other, and will conclude that their lawn or their soil is the problem.
A measured rate plus an observed runoff time
You ran the test and got 1.4 in/hr, and you watched a run and saw water reach the path at 8 minutes. The tool returns 21 minutes per watering day delivered as 3 cycles of 7 minutes each, with at least 60 minutes between start times, and explains why: a single run of 21 minutes would put roughly 0.3 in onto a surface that had already stopped accepting it. Programme three start times, not one long one.
Rain does the job for you
Weekly target one inch, and 1.2 inches of rain has fallen. The tool returns No irrigation needed this week and tells you to switch the controller off rather than let it run into wet soil, citing EPA on what frequent watering into saturated ground actually does — drowned plants, shallow roots, and pooling that drives weeds, disease, fungus and polluted runoff. This is the case a set-and-forget controller gets wrong every time it rains.
What changes the result
What actually moves the answer, ranked.
1. The precipitation rate. Nothing else is close. It is a straight divisor on the run time, and the plausible range across common domestic hardware — CSU’s ¼ to 2½ in/hr — spans a factor of ten. Every other input adjusts the answer; this one sets it.
2. The weekly depth. Linear, and the input most worth localising. Swapping our national one-inch default for your own district’s published weekly figure is the single best improvement you can make to this calculation.
3. The runoff observation. Changes not how much water is applied but whether it arrives in the soil or on the pavement. On clay and on slopes it is the difference between the schedule working and the schedule being theatre.
4. Rain. Subtracted directly, and capable of taking the answer to zero. Chronically ignored by controllers without a rain sensor.
5. Watering days. Divides the same weekly total into more or fewer sessions. It changes root behaviour more than it changes the water bill.
6. Zone overlap. A halving switch. Binary, easy to miss, and doubles the water on the overlap strip when missed.
What is deliberately absent.
No soil infiltration table. The conventional driver of cycle-and-soak, omitted because we could not verify one in a primary document we actually read on 5 September 2026. Your observed runoff minute replaces it and is a better measurement anyway.
No evapotranspiration model. The topic plan called for county gridded ET. We did not ship it: it answers a less binding question than the precipitation rate, and it cannot be verified by the reader standing in their own garden. Your water district’s weekly number is the local adjustment, from a body that measured it.
No grass-species coefficients. CSU’s figures are for cool-season turf, and it notes xeric and dry-land plants may need significantly less. We do not publish crop coefficients we have not individually sourced.
No cost or savings figure. Water rates vary by district and tier, and an invented saving would be the least defensible number on the page. Priced separately, at your own rate off your own bill, on the lawn water cost calculator.
Local considerations
Your district probably publishes the number you want. Water providers and conservation programmes across the country publish weekly watering guidance for their own service area, often updated through the season from local weather. Where that exists it is strictly better than a national inch-a-week default, and the calculator has a field for it. This is also why the page is built to be embedded: the arithmetic is universal, the weekly number is local, and the two should come from different places.
Soil, as context rather than as a coefficient. A parcel-level lookup of the mapped soil, its drainage class, its hydrologic soil group and its shrink-swell potential. It is free, it takes about five minutes, and it is the cheapest useful thing a homeowner can do before buying hardscape. It is a survey map, not a site investigation of your lot. Hydrologic soil group D — "Clays with high shrink-swell, a high water table, a claypan, or shallow soil over near-impervious material." — is fair warning to expect cycling. Group A — "Deep sands and gravelly sands. Water goes down." — usually is not. We use this to set expectations, never to compute a number.
Rules that are genuinely local. Watering-day restrictions, permitted hours, drought stages and rain-sensor requirements are set by state and district and change during a drought. CSU notes many states, though not Colorado, require rain shut-off sensors on automatic systems. Check your own provider before assuming a schedule is allowed.
When not to use this
Do not use it on a system whose cans filled unevenly. That is a hardware fault and no run time resolves it — you would only be choosing which parts of the lawn to sacrifice.
Do not use it for drip irrigation, bubblers or hand watering. Drip is measured in gallons per hour per emitter, not inches per hour over an area, and the arithmetic here does not apply.
Do not use it for new seed or new sod. Establishment watering is frequent and shallow by design — the opposite of everything on this page — and it is a horticultural decision, not a scheduling one.
Do not use it for warm-season grasses or drought-adapted planting without adjusting the weekly depth. CSU’s figures are for cool-season turf and the document says xeric and dry-land plants may need significantly less.
Do not treat the output as permission. Drought restrictions and watering-day rules override any calculator, including this one.
And do not treat it as a prescription. Every source on this page ends the same way: tune by watching the lawn, in 10 percent steps. HyreYard does not install, service or design irrigation, sells no controllers, and takes no fee from anyone who does.
Related on this site
- Lawn water cost calculator Prices the depth this page schedules, at your own water rate. Deliberately no cost figure here; deliberately no run time there.
- Turf rebate Sometimes the right answer is less irrigated lawn, not a better schedule.
- Drainage diagnostic For water that pools when the system is off.
- Landscaping cost estimator If irrigation sits inside a larger project.
Questions this calculator answers
- How long should I run my sprinklers?
- Long enough to apply the depth of water your lawn needs, which depends entirely on how fast your sprinklers apply water. EPA WaterSense suggests about one inch a week including rainfall as a starting point. At CSU Extension’s working estimate of 1.75 in/hr for pop-up spray heads, half an inch takes about 17 minutes; at 0.5 in/hr for rotors, the same half inch takes about 60 minutes. Measure your own rate with the six-can test on this page and the minutes stop being a guess.
- How do I measure my sprinkler precipitation rate?
- CSU Extension’s method: place six identical straight-sided, flat-bottomed cans randomly between the heads in one zone, run the zone for exactly ten minutes, pour all six cans into one, and measure the depth with a ruler. That depth in inches is your precipitation rate in inches per hour — six cans of ten minutes is sixty can-minutes, so the test converts itself. Do not use tuna cans; CSU warns they are too shallow and water splashes out. Repeat per zone, because the rate differs zone to zone.
- How much water does a lawn need per week?
- EPA WaterSense: "Your landscape will typically require one inch of water a week, including rainfall, and that can vary depending on where you live, recent weather, and the plants in your landscape." Treat one inch as a starting point rather than a rule. Many water districts publish a weekly watering figure for their own service area that updates through the season, and where one exists it is better than any national number — including ours.
- What is cycle and soak, and do I need it?
- It means splitting one long run into several short ones with soak-in time between. You need it when water is arriving faster than your ground will take it. CSU: "On slopes or compacted, clayey soils, water is generally applied faster than it can soak into the soil, resulting in water being wasted as it runs off-site." The test is direct — watch the run and see whether water pools or moves. If it never does, you do not need cycles and adding them is pure complication.
- How do I know how many cycles to use?
- This tool takes the minute at which you observed runoff beginning and splits the total run into cycles no longer than that, then spaces the start times at least an hour apart, per CSU. Most calculators instead look up an infiltration rate from a soil-texture table. We deliberately do not, because we could not verify such a table in any primary source we actually read on 5 September 2026 — and because your observed runoff minute is a direct measurement of your own soil, slope, compaction and rate, which a texture table only approximates.
- Should I water every day?
- Generally no. EPA notes watering too frequently "can drown plants or result in shallow roots". CSU’s guidance for Colorado soils is once a week in spring and autumn and twice a week in summer, with the observation that "watering as infrequently and deeply as the soil allows gives better resilience during hot spells and helps reduce many weed species." Fewer, longer waterings — split into cycles if they run off — beat daily sprinkles.
- What time of day should I water?
- CSU: "Many water providers encourage homeowners to water their yards between 9 p.m. and 9 a.m. Winds are typically less at night, and evaporation loss will be lower." Wind is the underrated half of that — it distorts the spray pattern as well as evaporating the water, so a windy midday run is uneven as well as wasteful.
- How do I know if I am overwatering?
- CSU gives a test most people find backwards: "In unusually hot weather, if dry spots do not pop up, the lawn is over-watered." A precisely tuned system should show some stress in a genuine heatwave. If yours never does, it is carrying a permanent surplus. The signal to water is colour shifting from bluish-green to grey-blue, with footprints still visible an hour or more later.
- Two of my zones water the same strip. Does that matter?
- Yes, and it is easy to miss. CSU: "In many lawn sections, one zone waters the area from the left while another zone waters the same area from the right. In this situation, cut run times for zones in half, so that each applies half of the needed water." There is a switch for this in the calculator. Without it, the overlap strip gets double every single time.
- My catch cans filled very unevenly. What does that mean?
- That the problem is not your schedule. CSU: "If the amount of water in some containers is significantly more or less than others, the system is poorly designed, or head(s) are malfunctioning." No run time fixes uneven distribution — you would be choosing between drowning the wet parts and starving the dry ones. Check for tilted or blocked heads, mismatched nozzles and pressure problems first.
- Do I need to change the schedule through the season?
- Yes, and it is the single biggest easy saving. CSU’s approach uses the percent key on the controller: set it for the July–August schedule, then run at "50%, 67% or 100%, based on the season". A spring setting left running into autumn waters nothing useful. Fine-tune in 10 percent increments rather than large jumps.
- Does this tool know my local evapotranspiration?
- No, and it says so rather than implying otherwise. The original plan for this page called for county-level gridded reference ET. We did not ship that: it would have to be frozen into the page, and more importantly it answers a question that is not the binding one. The number that actually sets your run time is your own precipitation rate, which no national dataset holds. Where your water district publishes a weekly watering figure, type it into the weekly-depth field — that is the local adjustment, from the body that measured it.
Sources and methodology
Figures dated 5 September 2026. Last reviewed .
- Watering Tips (US Environmental Protection Agency, WaterSense, retrieved 2026-09-05. Source of the one-inch-a-week including-rainfall starting point, the tuna-can timing method, the cycle-and-soak guidance for clay soils and slopes, and the overwatering consequences quoted on this page.)
- WaterSense outdoor water use (US Environmental Protection Agency, retrieved 2026-09-05. The ~8 billion gallons a day figure and the statement that as much as 50 percent is wasted through overwatering and system inefficiency.)
- CMG GardenNotes #265, Methods to Schedule Home Lawn Irrigation (Colorado State University Extension, retrieved 2026-09-05. Read in full. Source of the spray 1–2½ in/hr and rotor ¼–¾ in/hr ranges and their working estimates, the six-can ten-minute catch-can procedure, the uniformity warning, the overlapping-zone halving rule, the one-hour minimum between cycle start times, the 10 percent tuning increments, the 50/67/100 percent seasonal key and the dry-spots test for overwatering. Authors: David Whiting (retired); revised September 2017 by Kurt M. Jones; reviewed April 2023 by Marvin Reynolds.)
- Web Soil Survey (USDA Natural Resources Conservation Service, retrieved 2026-09-05. Free parcel-level lookup of mapped soil and hydrologic soil group. Cited as useful context for whether to expect runoff — not used as a source of infiltration rates, which we could not verify.)
- Simple Irrigation Audit for Home Lawns in Oklahoma (Oklahoma State University Extension, retrieved 2026-09-05. Listed because it is a good second reference for the audit method for readers who want one. RETRIEVAL NOTE: the server returned HTTP 403 to automated retrieval on 5 September 2026, so nothing on this page is quoted from it or sourced to it.)
Related
- Irrigation systems Where the efficiency actually comes from: zoning by plant need, a sensor and a schedule.
- Lawn water cost calculator The other half of this question. Take the depth you land on here and it prices the gallons at your own utility rate — including the tier and sewer traps that make irrigation cost more than the headline rate.
- Turf rebate If the honest answer is less lawn rather than better scheduling, this prices the swap at your own utility rate.
- Drainage diagnostic If water is pooling when the sprinklers are off, the problem is drainage, not run time.
- Landscaping cost estimator If irrigation is one line in a bigger project budget.
- Mulch calculator Mulched beds hold moisture and need a different schedule from turf.