Irrigation Run Time Calculator: A Worked Guide

The irrigation run time calculator turns a depth to apply, a precipitation rate and an efficiency figure into minutes per zone. Apply 12 mm at 10 mm per hour and 80 percent efficiency and each zone runs 90 minutes. Four zones watered three times a week add up to 18 hours of controller time.

Most watering advice gives a fixed number of minutes, and that number only works for the system it was written for. A spray zone finishes far sooner than a slow rotor zone for the same depth.

The depth comes first. Decide how much water the soil needs at each watering, then let the calculator turn that depth into minutes. The page example uses 12 mm, inside the usual 12 to 25 mm range.

What the calculator answers

The calculator answers two questions. How many minutes should each zone run at one watering. How much controller time does the whole week add up to. It asks for the depth to apply, the precipitation rate or the flow and area of the zone, the efficiency, the watering days each week and the number of zones.

The calculator gives no cost figure, no pump or pressure figure and no sprinkler hardware sizing. It also gives no precipitation rate for any named sprinkler brand or model.

How the irrigation run time calculator works

The page gives three formulas. The first finds the precipitation rate when it is not known. Precipitation rate equals flow times 60 divided by area. Flow is in litres per minute and area is in square metres.

One litre per minute over 1 square metre equals 60 mm per hour, because 1 mm of water over 1 square metre is 1 litre.

The second formula finds minutes per zone. Minutes equals depth divided by efficiency divided by precipitation rate times 60. Depth and rate must use the same units.

The third formula finds weekly time. Weekly time equals minutes times days times zones.

The worked example in full

The page example applies 12 mm at 10 mm per hour and 80 percent efficiency, and the answer is 90 minutes per zone.

Step 1, start with the depth, 12 mm. Step 2, allow for efficiency. Divide 12 by 0.80 to get 15 mm, the depth to apply so 12 mm stays in the soil after losses. Step 3, divide by the precipitation rate. Divide 15 by 10 to get 1.5 hours. Step 4, turn hours into minutes. Multiply 1.5 by 60 to get 90 minutes.

Check it backwards: 10 mm per hour for 90 minutes puts down 15 mm, and 80 percent of 15 mm is 12 mm. Without the efficiency step the answer would be 72 minutes, and that run would leave only 9.6 mm in the soil, short of the 12 mm target.

How depth changes run time

Run time rises in direct proportion to depth, so doubling the depth doubles the run time. The table keeps the page rate of 10 mm per hour and 80 percent efficiency, and changes only the depth.

Depth to apply	Minutes per zone
6 mm	45
9 mm	67.5
12 mm	90
18 mm	135
25 mm	187.5

A 25 mm watering at this rate takes more than three hours per zone, so a deep watering on a slow system often needs cycle and soak.

How precipitation rate changes run time

A faster system finishes sooner for the same depth, and halving the rate doubles the run time. The table keeps the page depth of 12 mm and 80 percent efficiency. Every rate below is an example input.

Precipitation rate example input	Minutes per zone
4 mm per hour	225
6 mm per hour	150
9 mm per hour	100
10 mm per hour	90
14 mm per hour	64.3
18 mm per hour	50

Sprays at 18 mm per hour water 12 mm in 50 minutes, while a rotor zone at 4 mm per hour needs 225 minutes.

How to find your precipitation rate

Two routes give the rate. The first is a cup test. Set straight sided cups across the zone, run the zone for 15 minutes, measure the average depth in the cups and multiply by 4. An average of 3 mm after 15 minutes is 12 mm per hour. An average of 2 mm is 8 mm per hour. Place cups across the whole zone, not only in the wettest spot.

The second route uses flow and area. Precipitation rate equals flow times 60 divided by area, and the table shows example inputs worked through that formula.

Flow example input	Area example input	Precipitation rate
6 litres per minute	30 square metres	12 mm per hour
9 litres per minute	30 square metres	18 mm per hour
12 litres per minute	40 square metres	18 mm per hour
10 litres per minute	100 square metres	6 mm per hour
20 litres per minute	80 square metres	15 mm per hour

Weekly controller time

Minutes per zone is only half the answer: the controller runs every zone on every watering day. Weekly time equals minutes times days times zones. The page example runs four zones three times a week at 90 minutes each: 90 times 3 is 270 minutes per zone per week, 270 times 4 is 1080 minutes, and 1080 divided by 60 is 18 hours. The table shows other combinations at 90 minutes per zone.

Zones	Watering days per week	Weekly controller time
3	2	540 minutes, 9 hours
5	2	900 minutes, 15 hours
4	3	1080 minutes, 18 hours
2	4	720 minutes, 12 hours
6	2	1080 minutes, 18 hours

Weekly time is controller time, not water volume. One more watering day for four zones at 90 minutes adds 6 hours to the week.

Why efficiency matters

Efficiency is the share of applied water that stays where plants can use it. The page default is 80 percent, and a guessed figure makes the minutes figure a guess too.

At 100 percent efficiency the same 12 mm takes 72 minutes. At 80 percent it takes 90 minutes. At 70 percent it takes about 103 minutes. At 60 percent it takes 120 minutes.

Even coverage can justify a higher figure. Dry patches, wind exposure or drifting spray deserve a lower one.

Sprays and rotors

The page gives typical run times for each head type. Spray heads usually run 15 to 30 minutes per watering of about 12 to 25 mm, rotors often 45 to 90 minutes. The gap comes from the rate: rotors apply slowly, often under 12 mm per hour, while sprays apply faster.

A rotor zone at 10 mm per hour needs 90 minutes for 12 mm, at the top of the rotor range. The ranges are typical, not fixed.

Cycle and soak on clay or slopes

Long runs on clay or slopes can run off before the water soaks in. The page answer is cycle and soak: split one long run into shorter ones with breaks, so water soaks in instead of running off.

A 90 minute run becomes, as an example split, three 30 minute cycles with a soak break between each one. The total and the depth stay the same. Clay absorbs slowly and a slope sends water downhill, so both benefit. If water is still on the surface when the next cycle starts, lengthen the break.

Imperial example

The page example is metric. The imperial example below is separate, with rounded inputs, and is a rounded equivalent, not an exact conversion.

Apply 0.5 inches at 0.4 inches per hour and 80 percent efficiency: 0.5 divided by 0.80 is 0.625 inches, 0.625 divided by 0.4 is 1.5625 hours, and 1.5625 times 60 is 93.75 minutes, shown as about 94 minutes.

Check it backwards. At 0.4 inches per hour for 94 minutes the system puts down about 0.63 inches, and 80 percent of that is about 0.5 inches.

Seasonal adjustment

Run times change with the seasons, and the page says to adjust them as weather changes. Hot, dry and windy weather dries soil faster, so the depth or the frequency rises. Cool and cloudy weather slows drying, so the run time or the watering days fall.

Change one thing at a time. Change the minutes first and watch the soil for a week, or change the watering days and keep the minutes fixed. The depth and the gap between waterings come from the irrigation scheduling calculator, and the crop water requirement calculator gives the daily water use behind the depth.

How to use the calculator

1. Enter the depth to apply at each watering.
2. Enter the precipitation rate, or the zone flow and area.
3. Leave efficiency at 80 percent unless the system has been tested.
4. Enter the watering days and the number of zones.
5. Set each zone to the minutes shown.
6. Use cycle and soak on clay or slopes.

Common mistakes

1. Using the depth as the run time. A 12 mm depth is not a 12 minute run.
2. Forgetting efficiency. A 72 minute run leaves only 9.6 mm in the soil.
3. Copying a neighbour minutes figure. Spray and rotor zones can differ by hours for the same depth.
4. Mixing metric and imperial units. A depth in millimetres with a rate in inches per hour gives a wrong answer.
5. Measuring the rate in the wettest spot only. Cups across the whole zone show the uneven coverage.
6. Running one long cycle on clay or a slope. Water that runs off never reaches the roots.
7. Setting the schedule once for the whole year. Hot and cool weather need different run times.

How this fits with other tools

This calculator turns a depth into minutes. The depth comes from the irrigation scheduling calculator linked above. The garden watering schedule calculator turns weekly need and rain into litres and sessions. The rain barrel calculator shows how much rain a roof can supply. The drip irrigation planner turns a depth into emitters, tubing and flow, and drip zones run longer than sprinkler zones because drip applies water slowly at the roots.

The calculator is at https://theplantcalculator.com/water-irrigation/irrigation-run-time-calculator/ and its worked example is the source for every page figure here.

Frequently asked questions

Q: How long should I run my sprinklers?
A: About 12 to 25 mm per watering. Sprays usually run 15 to 30 minutes, rotors often 45 to 90 minutes. Exact minutes depend on rate and efficiency.

Q: How do I find my precipitation rate?
A: Set straight sided cups in the zone, run it for 15 minutes, measure the average depth and multiply by 4. An average of 3 mm is 12 mm per hour. Flow and area give the same figure: rate equals flow times 60 divided by area.

Q: What is cycle and soak?
A: Splitting one long run into shorter ones with breaks, so water soaks in instead of running off. A 90 minute run becomes, as an example split, three 30 minute cycles with a soak break between each one. Use it on clay or slopes.

Q: Do drip zones run longer than sprinkler zones?
A: Yes. Drip applies water slowly at the roots, so run times are longer but less water is wasted.

Q: Why does 12 mm at 10 mm per hour take 90 minutes and not 72 minutes?
A: Because the system is 80 percent efficient: 12 divided by 0.80 is 15 mm, 15 divided by 10 is 1.5 hours, and 1.5 times 60 is 90 minutes. A 72 minute run would leave only 9.6 mm in the soil.