What a run time really is: depth, not duration
Irrigation is measured in depth, the same way rainfall is. When a weather report says half an inch fell, it means that if none of it soaked away or evaporated, water would stand half an inch deep on level ground. Plants care about depth because depth translates directly into how far water penetrates the soil profile: roughly one inch of water wets one foot of a loam soil, four to eight inches of a sand, and less than half a foot of a heavy clay.
Your controller, however, is programmed in minutes. The bridge between the two is the precipitation rate — how many inches per hour that particular zone puts down. A spray zone that delivers 1.6 in/hr and a rotor zone that delivers 0.35 in/hr apply the same half inch in wildly different times: about 19 minutes versus about 86 minutes. Running every zone for the same number of minutes, which is what most controllers ship with, guarantees that some zones are drowned and others are starved.
That is the entire job of this calculator. It converts the depth you want into the minutes your controller needs, using the actual output of the zone in front of you.
How the 96.3 formula and the efficiency term work
Start with the precipitation rate. You know the zone's flow in gallons per minute and the area it waters in square feet, and you want inches per hour. A US gallon is 231 cubic inches, there are 60 minutes in an hour, and a square foot is 144 square inches, so the conversion factor is 231 × 60 ÷ 144 = 96.25. The trade rounds this to 96.3, and that is where the famous constant comes from — it is a unit conversion, not an empirical fudge.
So PR = 96.3 × Q ÷ A. A zone flowing 12 GPM over 1,500 ft² has a precipitation rate of 96.3 × 12 ÷ 1,500 = 0.770 in/hr. In metric the same idea is simpler: litres per hour divided by square metres gives millimetres per hour directly, with no constant at all.
The second term is application efficiency. Not every gallon that leaves a nozzle ends up stored in the root zone where you want it. Some drifts, some evaporates in flight, some lands on pavement, and — the biggest loss on most systems — some falls on the parts of the zone that are already wet because the pattern is uneven. Dividing by an efficiency of 0.75 stretches the run time by a third so that the driest part of the zone still receives the depth you asked for. Set it to 100% and you are calculating the average depth, which leaves the dry corners short.
Efficiency and uniformity are related but not identical. Uniformity describes how evenly water lands; efficiency describes how much of what you pumped is useful. A zone with poor uniformity can only be made adequate by over-watering the good areas, so low uniformity forces low efficiency. Measure yours with a catch-can test before you trust any assumed number.
Worked example: a 1,500 ft² rotor zone flowing 12 GPM
You have a lawn zone with four gear-drive rotors, each rated 3.0 GPM at the pressure you measured, covering 1,500 ft². You want to apply half an inch, your soil is a loam that takes water at about 0.5 in/hr, and you assume 75% application efficiency.
- Total the flow. 4 rotors × 3.0 GPM = 12 GPM.
- Convert to a precipitation rate. PR = 96.3 × 12 ÷ 1,500 = 0.770 in/hr.
- Apply the efficiency. Useful rate = 0.770 × 0.75 = 0.578 in/hr.
- Convert depth to time. t = 60 × 0.50 ÷ 0.578 = 51.9 minutes.
- Check against the soil. The zone puts down 0.770 in/hr but the loam only takes 0.5 in/hr, so a single 52-minute run will pond. Split it into two cycles of 26 minutes with at least an hour of soak between them.
- Work out the water used. The gross depth pumped is 0.50 ÷ 0.75 = 0.667 in. Gallons = 0.667 × 1,500 × 0.6234 = 623 gallons. Cross-check with the flow: 12 GPM × 51.9 min = 623 gallons. The two agree, which is the arithmetic proof that the 96.3 constant and the 0.6234 constant are the same conversion running in opposite directions.
- Build the week. If the turf needs 1.0 inch a week and you water twice, each day needs 60 × 1.0 ÷ 0.578 ÷ 2 = 52 minutes — the same figure, because half an inch twice a week is one inch a week.
How to read the result and set the controller
The first thing to check is whether the precipitation rate exceeds the soil's intake rate. If it does, the run time the calculator gives you is still the right total, but you cannot deliver it in one go. Cycle-and-soak splits the total into shorter starts separated by a soak period long enough for the surface water to move down — an hour is a reasonable default, and most controllers can do this automatically with a repeat or cycle+soak setting.
Second, look at the total against the calendar. Deep, infrequent irrigation grows deep roots; light daily sprinkles grow shallow ones and waste water to evaporation. If the calculator says 52 minutes twice a week and your controller is currently set to 15 minutes every day, you are applying similar water with much worse results. Fewer, longer runs is almost always the improvement.
Third, sanity-check the run against the season. Turf and most row crops need somewhere near their evapotranspiration rate, which in midsummer in a hot dry climate can approach 0.3 in/day and in a cool coastal one may be a third of that. Rather than guess, take the number from a crop water requirement calculation or a reference ET figure for your area and put it in the weekly target field.
Finally, treat the gallons figure as a budget line. A single 623-gallon zone run twice a week is 64,800 gallons a season over 26 weeks. Multiply by the number of zones and you have the number that appears on the water bill.
Minutes to apply a given depth at common precipitation rates
| Precipitation rate (in/hr) | 0.5 in at 100% | 1.0 in at 100% | 0.5 in at 75% | 1.0 in at 75% |
|---|---|---|---|---|
| 0.25 | 120 | 240 | 160 | 320 |
| 0.35 | 86 | 171 | 114 | 229 |
| 0.50 | 60 | 120 | 80 | 160 |
| 0.75 | 40 | 80 | 53 | 107 |
| 1.00 | 30 | 60 | 40 | 80 |
| 1.50 | 20 | 40 | 27 | 53 |
| 2.00 | 15 | 30 | 20 | 40 |
Gear rotors typically land between 0.3 and 0.5 in/hr, fixed spray heads between 1.3 and 2.0 in/hr — which is why a spray zone and a rotor zone must never share a run time.
Basic soil intake rates by texture
| Soil texture | Basic intake (mm/hr) | Basic intake (in/hr) |
|---|---|---|
| Sand | more than 30 | more than 1.2 |
| Sandy loam | 20–30 | 0.8–1.2 |
| Loam | 10–20 | 0.4–0.8 |
| Clay loam | 5–10 | 0.2–0.4 |
| Clay | 1–5 | 0.04–0.2 |
Cut these figures roughly in half on slopes steeper than about 5%, on compacted turf, and on soil with a thatch or crust layer. Inches per hour are the millimetre figures divided by 25.4.
Measure the flow instead of trusting the catalogue
Nozzle flows are quoted at a stated pressure. If your zone runs at 35 psi instead of the 45 psi in the chart, a rotor's discharge falls by roughly 12%, because flow through an orifice varies with the square root of pressure. The fastest honest measurement is the water meter: shut everything else off, run the zone, and read the meter over exactly one minute. A meter reading in cubic feet converts at 7.48 gallons per cubic foot.
Mistakes that make a run time wrong
- Using the whole lawn area instead of the zone's area. The formula wants the ground that this zone wets. Using the property area inflates the run time in direct proportion to the error.
- Mixing spray heads and rotors on one zone. Their precipitation rates differ by a factor of four or more, so no single run time is correct for both. Rezone, or fit matched-precipitation rotary nozzles.
- Assuming part-circle heads apply the same rate as full-circle ones. A half-circle head pushing the same flow into half the area doubles the rate unless the nozzle is matched-precipitation.
- Ignoring runoff. Water that leaves the zone was pumped but never counted; the depth you calculated never reached the root zone. Split into cycles whenever the rate exceeds the soil's intake.
- Setting efficiency to 100%. That computes the average depth, so roughly half of the zone receives less than you intended.
- Forgetting rainfall. Subtract effective rain from the weekly target before you set the schedule, or fit a rain sensor, which most jurisdictions now require on new systems.
- Watering at midday in wind. Wind drift and evaporation losses are what push efficiency down; an early-morning start recovers several percentage points at no cost.
Where this fits among scheduling methods
There are three ways to decide when and how long to irrigate, and this calculator handles the arithmetic for all of them.
Fixed schedule. You choose a depth and a frequency and stick to it. Simple, and adequate for turf in a stable climate, but it over-waters in cool weeks and under-waters in heat waves.
ET-based (checkbook) scheduling. You track crop water use day by day, subtract effective rainfall, and irrigate when the running deficit reaches the depth you are willing to let the soil dry down. This is the method extension services and the FAO recommend, and it is what the ETc calculator is for. The run time still comes from the formula on this page.
Soil-moisture based. Sensors or a soil probe tell you the actual depletion and you irrigate to refill it. The most accurate approach, and the one that catches the errors in the other two, but it needs hardware and calibration.
Drip and micro-irrigation are a different arithmetic entirely: emitters are rated in gallons per hour and wet only part of the surface, so precipitation rate is a much less useful concept. Use the drip flow and run time calculator for those zones. If you are sizing a supply rather than a schedule, the pond volume calculator tells you how many acre-feet you have to draw on, and the field acreage calculator converts an odd-shaped block into the area figure this page needs.
Key terms
- Precipitation rate
- The depth of water a sprinkler zone applies per hour, in inches per hour or millimetres per hour. The single most important number about any zone.
- Application efficiency
- The fraction of water pumped that ends up stored in the root zone of the area being irrigated. Losses are drift, evaporation, runoff, deep percolation and non-uniformity.
- Distribution uniformity (DU)
- The average depth caught in the driest quarter of a catch-can grid divided by the average depth over the whole grid. Reported as a decimal or a percentage.
- Cycle and soak
- Splitting one irrigation into several short starts with soak periods between them, so the application rate does not outrun the soil's intake rate.
- Acre-inch
- One inch of water over one acre: 3,630 ft³, or 27,154 US gallons. Twelve acre-inches make an acre-foot.