SettingsSettings for this calculationUS
Work one head's square of ground, or the whole valve zone at once.
The head-spacing method is the designer's: it gives the rate inside a regular grid of matched heads before anything is installed. The total-area method suits a zone that already exists, where the valve's flow is known from a meter or by adding up the nozzles, and the area is measured; it averages over edges, odd corners and mixed heads that a single square cannot show.
The nozzle's flow at the pressure the zone runs at, from the maker's performance table.
Read the row for the operating pressure at the head, not the top of the table. Enter the flow of the head as set, together with its arc below: a half-circle head rated 1 gal/min (3.79 L/min) puts as much water on its half-circle as a 2 gal/min (7.57 L/min) full circle does on a whole one, and the page scales it up. Matched-precipitation nozzles are made so that this works out the same for every arc.
Tools needed: The nozzle maker's performance table
360 for a full circle, 180 for a half, 90 for a quarter; the page scales the flow to a full circle.
The rate depends on the flow per degree of arc, so a head's flow is multiplied by 360 and divided by its arc to give the full-circle flow the formula wants. A zone mixing heads whose flow does not follow their arc (unmatched nozzles) has no single rate: work each kind of head, or use the total-area method.
The distance between neighbouring heads along a row, S in the formula.
Head-to-head spacing puts each head where its neighbour's water reaches, so the spacing is about the radius of throw at the design pressure; the page's head spacing calculator sets it from the radius and the wind. A wider spacing spreads the same flow over more ground and lowers the rate with the square of the increase.
Tools needed: Tape measure, Layout drawing
Whether alternate rows sit opposite each other or offset by half a spacing.
In a triangular grid every head is the same distance from its neighbours in the next row, which puts the rows closer together than the heads: 0.866 times the head spacing. The page uses that in place of the row spacing box, which then only matters for square and rectangular layouts.
The distance between one row of heads and the next, L in the formula; equal to the head spacing for a square grid.
Measure square to the rows. A rectangular grid often closes the spacing across the prevailing wind and opens it along the wind, which is why the two spacings are entered separately. For a triangular layout the page works the row spacing out itself and ignores this box.
The depth the planting needs each week in the driest season, rain and all other water left out.
This is the irrigation requirement the schedule has to meet, usually from local evapotranspiration data for the plants and the season. The page opens on the 1-1/2 in (38 mm) a week Rain Bird's design manual works its own examples with; it is an example, not a recommendation for any plant or place.
How many days a week the zone may run, under the local rules and the site's use.
Fewer watering days means a longer run each day for the same weekly depth. Where water authorities restrict days, or a playing field can only be watered at night, this is the number the schedule has to fit.
Precipitation rate
0.856 in/h
The rate is the flow that falls between neighbouring heads divided by the ground between them: a full-circle head waters a quarter of each square it bounds, so one head's full-circle flow over one head spacing times one row spacing is the zone's rate, if every head is matched to its arc.
- Ground the flow is spread over
- 225 ft²
- Flow falling on that ground
- 2 gal/min
- Row spacing used
- 15 ft
- Depth applied in a 10-minute run
- 0.14 in
- Run time per watering day for the week's water
- 35.06 min
They open the calculator with your figures already in it
Sprinkler Precipitation Rate Calculator (in/h and mm/h): 0.8556 in/h — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Rain Bird Corporation, Landscape Irrigation Design Manual, pp. 43–46: PR = 96.3 x gpm / (S x L) in inches per hour and PR = 1000 x m3/h / (S x L) in millimetres per hour, gpm the total flow applied to the area by the sprinklers, S the head spacing and L the row spacing; 96.3 = 231 in³ per gallon / 144 in² per square foot x 60 minutes; four full-circle heads each put a quarter of their flow into the square between them, so one head's discharge is the flow applied; part-circle flows are added the same way; triangular spacing worked with L = S x 0.866; worked examples at 40 x 40 ft (12 x 12 m), 11 x 12 ft (3 x 4 m) and 70 ft (21 m) triangular
- The same manual, pp. 58–59: the circuit precipitation rate over a valve's area, and operating time OT = I x 60 / (PR x DA) in minutes a day, I the weekly irrigation requirement and DA the days available; worked at 1-1/2 in (38 mm) a week over 3 days
Inputs used
- Method
- Head spacing: one head's flow over the ground between heads
- Flow of One Head
- 2 gal/min
- Arc of That Head (degrees)
- 360
- Head Spacing Along a Row
- 15 ft
- Layout
- Square or rectangular: rows at the row spacing entered
- Row Spacing (square or rectangular layouts)
- 15 ft
- Total Flow of the Zone
- 16 gal/min
- Area the Zone Waters
- 1610 sq ft
- Water Needed per Week
- 1.5 in
- Watering Days per Week
- 3
Intermediate steps
- Ground the flow is spread over
- 225 ft²
- Flow falling on that ground
- 2 gal/min
- Row spacing used
- 15 ft
- Depth applied in a 10-minute run
- 0.14 in
- Run time per watering day for the week's water
- 35.06 min
Confidence note: The rate is the flow that falls between neighbouring heads divided by the ground between them: a full-circle head waters a quarter of each square it bounds, so one head's full-circle flow over one head spacing times one row spacing is the zone's rate, if every head is matched to its arc.
What this calculation does not cover
- A rate on paper, at the flows entered. Pressure at the heads below the table's row, worn or mismatched nozzles, and wind all change what lands; a catch-can test on the finished zone measures the rate that matters, and the page's catch-can calculator works it out.
- Average, not uniform. Every sprinkler throws more water close in than at the edge of its pattern, so the depth varies inside each square; low-quarter distribution uniformity measures how much, and a schedule meant to keep the driest ground alive runs longer than this average implies.
- The run time assumes the soil takes the water as fast as it arrives. On clay, or on a slope, a rate above the soil's intake rate runs off before it soaks in, and the usual answer is several short cycles with soak time between them rather than one long run; the soil's intake rate is not something this page knows.
- Rain, evaporation from the spray, and plants with different needs on one valve are outside the arithmetic: the weekly depth entered is taken to be what the planting needs from irrigation alone.
Add the equipment this sizes
This result is a specification — 0.856 in/h — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-10-05 · v1.0.0
Regulatory standards & verification citations2
- Rain Bird Corporation, Landscape Irrigation Design Manual, pp. 43–46: PR = 96.3 x gpm / (S x L) in inches per hour and PR = 1000 x m3/h / (S x L) in millimetres per hour, gpm the total flow applied to the area by the sprinklers, S the head spacing and L the row spacing; 96.3 = 231 in³ per gallon / 144 in² per square foot x 60 minutes; four full-circle heads each put a quarter of their flow into the square between them, so one head's discharge is the flow applied; part-circle flows are added the same way; triangular spacing worked with L = S x 0.866; worked examples at 40 x 40 ft (12 x 12 m), 11 x 12 ft (3 x 4 m) and 70 ft (21 m) triangular
- The same manual, pp. 58–59: the circuit precipitation rate over a valve's area, and operating time OT = I x 60 / (PR x DA) in minutes a day, I the weekly irrigation requirement and DA the days available; worked at 1-1/2 in (38 mm) a week over 3 days
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