Precipitation Rate Explained: Flow, Area and the Cup Test

Precipitation rate is the depth of water a sprinkler zone applies in one hour, measured in millimetres per hour. It equals flow times 60 divided by area. One litre per minute over one square metre gives 60 millimetres per hour. Measure it with straight sided cups, run the zone for 15 minutes, and multiply the average depth by four.

Flow alone does not tell you how wet the soil gets. A large volume spread over a large area still gives a thin depth, so two zones with the same flow can need very different run times.

What Precipitation Rate Means

Precipitation rate is the depth of water applied to a zone in one hour, measured in millimetres per hour in metric work and in inches per hour in imperial work. The word precipitation is borrowed from rainfall: a depth of water over an area, not a volume in a tank.

A litre is a volume, while a millimetre of depth is a volume spread over an area. One litre spread over one square metre gives a depth of one millimetre. That link is the whole idea behind the rate.

Pour the same volume over a small bed and the depth is deep. Pour it over a bed four times as large and the depth is one quarter as deep. Depth is what roots use, so a depth per hour decides how long the zone must run.

The Precipitation Rate Formula

Precipitation rate equals flow times 60 divided by area. Flow is in litres per minute. Area is in square metres. The answer is in millimetres per hour. The Irrigation Run Time Calculator uses this formula when the rate is not known, so you can enter flow and area instead.

The 60 converts minutes to hours: flow times 60 gives litres per hour, and litres per hour divided by square metres gives millimetres per hour, because one litre over one square metre is one millimetre deep.

The unit fact on the calculator page follows the same steps: one litre per minute over one square metre gives 60 millimetres per hour, because one litre per minute times 60 is 60 litres per hour over one square metre.

Area is the area the zone actually waters, not the size of the whole garden. Too large an area gives too low a rate, and too small an area gives too high a rate. Litres per minute goes with square metres, and the calculator gives no conversion figure for other flow units.

Worked Examples of Flow and Area

The table below works several flow and area pairs through the same formula. Each row except the first is an example input only. The first row is the verified unit fact from the calculator page.

Example flow	Example area	Precipitation rate
1 L per minute	1 m2	60 mm per hour
5 L per minute	10 m2	30 mm per hour
10 L per minute	10 m2	60 mm per hour
10 L per minute	20 m2	30 mm per hour
12 L per minute	15 m2	48 mm per hour
20 L per minute	20 m2	60 mm per hour
8 L per minute	40 m2	12 mm per hour

Two patterns show up at once. Doubling the flow while the area stays the same doubles the rate, and doubling the area while the flow stays the same halves the rate. Five litres per minute over 10 square metres gives 30 mm per hour, while 10 litres per minute over the same area gives 60 mm per hour.

The last row is worth a close look. Eight litres per minute sounds like a useful flow, yet over 40 square metres it gives only 12 mm per hour, so that zone needs a long run to build depth.

Same Flow, Different Area

Two zones can share one flow and still have very different precipitation rates if their areas differ. The table below holds flow fixed at 12 litres per minute, an example input, and changes only the area.

Example area	Precipitation rate
10 m2	72 mm per hour
20 m2	36 mm per hour
30 m2	24 mm per hour
40 m2	18 mm per hour

The smallest zone applies water four times as fast as the largest, even though both receive the same flow. The rate measures what lands per square metre, not what leaves the tap.

One run time cannot serve every zone. Set all four zones above to the same minutes and the smallest would receive four times the depth of the largest.

How Precipitation Rate Feeds Run Time

Precipitation rate feeds the minutes formula: minutes equals depth divided by efficiency divided by precipitation rate times 60. Depth is in millimetres, and efficiency is written as a decimal, so 80 percent becomes 0.80.

Efficiency defaults to 80 percent on the calculator page, so leave it there unless you have tested your system. An efficiency below 100 percent accounts for uneven coverage, wind and evaporation.

The verified anchor is 12 mm at 10 mm per hour and 80 percent efficiency, which gives 90 minutes: 12 divided by 0.80 is 15, 15 divided by 10 is 1.5, and 1.5 times 60 is 90.

The table below keeps depth at 12 mm and efficiency at 80 percent, and varies only the precipitation rate. Every rate except 10 mm per hour is an example input.

Precipitation rate	Minutes to apply 12 mm at 80 percent efficiency
6 mm per hour	150 minutes
8 mm per hour	112.5 minutes
10 mm per hour	90 minutes
12 mm per hour	75 minutes
15 mm per hour	60 minutes
20 mm per hour	45 minutes

A rate of 20 mm per hour needs less than one third of the time of a rate of 6 mm per hour for the same depth. The page also gives a weekly figure from the same anchor: four zones at 90 minutes each, three times a week, give 1080 minutes, which is 18 hours of controller time.

The Cup Test Step by Step

The cup test is the calculator page method for finding precipitation rate by measuring what actually lands. It needs straight sided cups. Straight sides matter, because a cup that narrows towards the base makes the same volume look deeper than it is.

Set the cups out across the zone, spaced evenly, with some near the edges and some near the middle. Do not put them all in one wet spot.

Run the zone for 15 minutes and measure the depth in each cup in millimetres. Average the readings, then multiply the average by 4 to get millimetres per hour, because 15 minutes is one quarter of an hour.

The table below shows example average readings and the rate each one gives. These are example readings only, not page figures.

Example average depth after 15 minutes	Precipitation rate
2 mm	8 mm per hour
2.5 mm	10 mm per hour
3 mm	12 mm per hour
4 mm	16 mm per hour
5 mm	20 mm per hour

If the cups differ a lot, coverage is uneven. The average still gives the rate for the zone as a whole, but the driest cups show the spots that will stay short. If the cup test and the flow and area formula disagree by a large amount, trust the cups and check the flow and area inputs.

Rotors and Sprays

Rotors apply slowly, often under 12 mm per hour. Sprays apply faster. That contrast explains why rotor zones often run for 45 to 90 minutes while spray zones often run for 15 to 30 minutes, the typical times in the page FAQ.

A slow rate is not a fault. It gives water time to soak in, which suits a zone where fast water would run off. It only means a longer run: a rotor zone at the anchor rate of 10 mm per hour needs 90 minutes for 12 mm at 80 percent efficiency, while a spray zone at an example rate of 20 mm per hour needs 45 minutes.

The calculator gives no precipitation rate for any named sprinkler brand or model, so do not use a rate from a box or another garden. Measure your own zone with the cup test, or work the rate from your own flow and area.

A Separate Rounded Imperial Example

The worked example on the calculator page is metric. The example below is separate, worked in inches, and it is a rounded imperial example, not an exact conversion.

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

Rotors often apply under 0.5 in per hour, which matches the metric note of under 12 mm per hour. The cup test works in inches too: average depth in inches after 15 minutes times 4 gives inches per hour.

Common Mistakes

1. Judging a zone by flow alone. Flow tells you what leaves the tap, while precipitation rate tells you what lands per square metre.
2. Using the wrong area. Area is the area the zone actually waters, not the whole garden.
3. Forgetting efficiency. Dividing depth by precipitation rate alone leaves out losses, so the stored depth ends short.
4. Mixing metric and imperial numbers in one sum. Litres per minute goes with square metres, and inches go with inches per hour.
5. Running the cup test badly. Cups that are not straight sided, or cups bunched in one wet spot, give a false average. Spread the cups out, use several, and run for 15 minutes before multiplying by 4.
6. Using a rate from another system. The calculator gives no rate for any named brand or model, so measure your own zone.

When to Measure Again

Measure the rate again whenever the zone changes. New heads, moved heads, a changed flow or a changed watered area all change the answer. The calculator page also says to adjust run times seasonally as weather changes.

Use the Irrigation Run Time Calculator again with the new rate, and set each zone to the minutes shown. If the depth you need has changed as well, the Irrigation Scheduling Calculator can set the depth and the gap between waterings. Write the flow, the area, the rate and the minutes down each time.

Frequently asked questions

Q: What is precipitation rate?
A: Precipitation rate is the depth of water a sprinkler zone applies in one hour, measured in millimetres per hour. It equals flow times 60 divided by area.

Q: How do you calculate precipitation rate from flow and area?
A: Multiply flow in litres per minute by 60, then divide by area in square metres. An example input of 10 litres per minute over 20 square metres gives 30 mm per hour.

Q: What does 1 litre per minute over 1 square metre equal?
A: It equals 60 mm per hour: one litre per minute times 60 is 60 litres per hour over one square metre.

Q: How does the cup test work?
A: Set straight sided cups across the zone, run it for 15 minutes, measure the average depth, and multiply by 4. An example average of 2.5 mm gives 10 mm per hour.

Q: Why do two zones with the same flow have different precipitation rates?
A: Their areas differ. The rate spreads flow over area, so a zone with twice the area gets half the rate from the same flow.

Q: How does precipitation rate change run time?
A: Minutes equal depth divided by efficiency divided by precipitation rate times 60. In the verified anchor, 12 mm at 10 mm per hour and 80 percent efficiency takes 90 minutes.