Sprinkler Coverage Calculator: A Worked Guide

A sprinkler coverage calculator shows how much ground one head covers, how fast water lands, and how long to run for a depth. A full circle head throwing 5 m covers 78.5 square metres, applies 24 mm per hour at 10 litres per minute on 5 m by 5 m spacing, and needs 30 minutes to apply 12 mm.

The rest of this guide shows where each number came from. Every figure after the worked example is an example input worked through the same formulas, with no figure from a named brand or model.

What the sprinkler coverage calculator works out

The calculator takes throw radius, arc, flow and spacing, and gives three outputs: coverage area for one head, precipitation rate and run time for the depth you enter. Coverage area is how much ground one head can reach alone. Precipitation rate is how fast water lands, in millimetres per hour or inches per hour. Run time is how long to leave the zone on to apply the depth you want.

The calculator assumes matched precipitation nozzles, so heads in one zone apply at a similar rate. Mixed nozzles apply unevenly, and wind and low pressure reduce real throw.

The formulas in plain words

Coverage of one head equals pi times radius squared times arc divided by 360. Pi is about 3.14159. Radius is throw distance from the head to the edge of the wet circle. Arc is the part of the circle the head waters, in degrees. A full circle is 360 degrees, a half circle is 180 degrees and a quarter circle is 90 degrees.

Precipitation rate in metric units equals flow times 60 divided by head spacing times row spacing. Flow is in litres per minute, spacing is in metres and the answer is in millimetres per hour. In US units, precipitation rate equals 96.25 times flow in gallons per minute divided by head spacing times row spacing in feet, and the answer is in inches per hour. Run time equals depth divided by precipitation rate, using the same units for both. Triangular row spacing equals head spacing times 0.866.

Worked example step 1: coverage area

A full circle head throws 5 m, so radius is 5 m and arc is 360 degrees. The arc factor is 360 divided by 360, which is 1. Coverage equals pi times 5 squared times 1. Five squared is 25. Pi times 25 is 78.54, which rounds to about 78.5 square metres.

That is the area of the full wet circle if the head stood alone. In a real layout heads overlap, so each head serves less than its full circle. A 5 m by 5 m layout gives each head a square of 25 square metres to serve, even though its wet circle covers 78.5 square metres. Overlap makes coverage even. A half circle head with the same throw covers about 39.3 square metres, and a quarter circle head covers about 19.6 square metres. The table below shows other example throw distances, worked through the same formula. All radii here are example inputs, not figures from a named product.

Throw radius	Full circle area	Half circle area	Quarter circle area
3 m	28.3 square metres	14.1 square metres	7.1 square metres
4 m	50.3 square metres	25.1 square metres	12.6 square metres
5 m	78.5 square metres	39.3 square metres	19.6 square metres
6 m	113.1 square metres	56.5 square metres	28.3 square metres
7 m	153.9 square metres	77.0 square metres	38.5 square metres

Worked example step 2: precipitation rate

The worked example uses flow of 10 litres per minute and spacing of 5 m by 5 m. Precipitation rate equals 10 times 60 divided by 5 times 5. Ten times 60 is 600. Five times 5 is 25. Six hundred divided by 25 is 24. The precipitation rate is 24 mm per hour.

Widen spacing and the rate falls. The table below keeps flow at 10 litres per minute and changes square spacing only. All spacings are example inputs.

Square spacing	Calculation	Precipitation rate
4 m by 4 m	600 divided by 16	37.5 mm per hour
5 m by 5 m	600 divided by 25	24.0 mm per hour
6 m by 6 m	600 divided by 36	16.7 mm per hour
7 m by 7 m	600 divided by 49	12.2 mm per hour
8 m by 8 m	600 divided by 64	9.4 mm per hour

Moving from 5 m by 5 m to 6 m by 6 m drops the rate to 16.7 mm per hour, a fall of almost a third. Flow changes the rate in direct proportion, so double flow doubles the rate.

Worked example step 3: run time

The worked example applies 12 mm at 24 mm per hour. Run time equals 12 divided by 24, which is 0.5 hours. Half an hour is 30 minutes. The table below uses the worked rate of 24 mm per hour only. All depths are example inputs.

Depth to apply	Calculation	Run time
6 mm	6 divided by 24	15 minutes
8 mm	8 divided by 24	20 minutes
10 mm	10 divided by 24	25 minutes
12 mm	12 divided by 24	30 minutes
18 mm	18 divided by 24	45 minutes
24 mm	24 divided by 24	60 minutes

The calculator does not set depth for you. For a fuller run time tool, the irrigation run time calculator works from the same idea, depth divided by rate.

Imperial example: a rounded equivalent

The imperial example is separate from the metric example, and it is a rounded equivalent, not an exact conversion. It uses flow of 2.5 gallons per minute and spacing of 15 ft by 15 ft. Precipitation rate equals 96.25 times 2.5 divided by 15 times 15. Ninety six point two five times 2.5 is 240.625. Fifteen times 15 is 225. Two hundred forty point six two five divided by 225 is 1.069, which rounds to 1.07 inches per hour.

That is the verified imperial figure. It sits just above the top of the range most lawns take without runoff, which the calculator page gives as 0.5 to 1 inch per hour. The metric worked rate of 24 mm per hour sits near the top of the metric range of 12 to 25 mm per hour for the same reason. The table below shows how the imperial rate changes with spacing at 2.5 gallons per minute. All spacings are example inputs.

Square spacing	Calculation	Precipitation rate
12 ft by 12 ft	240.625 divided by 144	1.67 inches per hour
15 ft by 15 ft	240.625 divided by 225	1.07 inches per hour
18 ft by 18 ft	240.625 divided by 324	0.74 inches per hour
20 ft by 20 ft	240.625 divided by 400	0.60 inches per hour

Square spacing versus triangular spacing

In a square layout, row spacing equals head spacing. Heads sit at the corners of squares. In a triangular layout, heads are staggered. Each row is offset by half a head spacing, so heads form triangles instead of squares. Row spacing is then head spacing times 0.866. For head spacing of 5 m, triangular row spacing is 5 times 0.866, which is 4.33 m. For 6 m it is 5.20 m, and for 4 m it is 3.46 m. Those are example inputs worked through the same factor.

Triangular spacing gives more even coverage, because staggered heads fill the gaps that square corners leave. Square spacing is easier to mark out.

The rate formula uses row spacing, so triangular spacing changes the rate even when head spacing stays the same. Triangular spacing uses 5 m by 4.33 m. The rate is 600 divided by 5 times 4.33, which is 600 divided by 21.65, or about 27.7 mm per hour. That faster rate needs a shorter run time for the same depth.

Comparing the rate with soil intake

A precipitation rate is only useful when you compare it with how fast soil can take water in. If water lands faster than soil can absorb it, the extra water runs off, pools in low spots, or flows away from roots. The calculator page gives one range for this comparison: most lawns take 12 to 25 mm per hour, or 0.5 to 1 inch per hour, without runoff, and clay soils prefer the lower end. The worked metric rate of 24 mm per hour sits near the top of that range. The imperial rate of 1.07 inches per hour sits just above the top of the imperial range. Both are at the fast end, and both need care on clay.

The calculator gives no soil infiltration rate figures, so this guide gives none either. It does not invent a rate for clay, loam or sand. The catch cup test finds your real rate. The garden watering schedule calculator helps plan days between runs.

The catch cup test

The catch cup test checks the real precipitation rate on your lawn, after wind, pressure and nozzle wear have had their say. Place straight sided cups across the lawn, in the zone you want to test. Spread them across the zone, not in one spot, so you catch the even and uneven parts.

Run the zone for 15 minutes. Measure the average depth in the cups. The average matters more than any single cup, because a single cup can sit in a wet spot or a dry spot. Multiply the average depth by 4 for the hourly rate. Fifteen minutes is a quarter of an hour, so multiplying by 4 gives the rate per hour. If the average depth after 15 minutes is 6 mm, the hourly rate is 24 mm per hour, which matches the worked example. If the average is 3 mm, the rate is 12 mm per hour.

Common mistakes

First, do not read coverage area as the area one head serves alone. A 5 m throw covers 78.5 square metres as a full circle, but in a 5 m by 5 m layout each head serves a 25 square metre square, with overlap from neighbours. Second, do not use throw from a nozzle chart without checking flow at that arc. Enter throw radius and arc from the nozzle chart, then enter flow at that arc. Third, do not mix nozzles in one zone. Matched precipitation nozzles apply at a similar rate, while mixed nozzles apply unevenly. Fourth, do not ignore wind and pressure. Wind shortens throw on the windward side, and low pressure shortens throw all round. Fifth, do not set a long run because the depth looks small. At 24 mm per hour, 12 mm needs only 30 minutes, while at 12 mm per hour it needs 60 minutes. Sixth, do not mix units in one calculation. Keep metric with metric and imperial with imperial, and label each result with its units. For head count, the sprinkler calculator covers that step.

FAQs

Q: What is a good precipitation rate for lawns?
A: Most lawns take 12 to 25 mm per hour, or 0.5 to 1 inch per hour, without runoff. Clay soils prefer the lower end. The worked rate here is 24 mm per hour, near the top, so it needs care on clay.

Q: How much area does a sprinkler cover?
A: A full circle head covers pi times radius squared. A 5 m throw covers 78.5 square metres, and a 15 ft throw covers about 707 square feet. Heads overlap in a layout, so each head serves less than its full circle.

Q: Why use triangular spacing?
A: Triangular spacing gives more even coverage, because staggered heads fill gaps that square corners leave. Row spacing is head spacing times 0.866, so rows sit closer together than in a square layout.

Q: How does a catch cup test work?
A: Place straight sided cups across the lawn, run the zone for 15 minutes, measure the average depth, and multiply by 4 for the hourly rate. An average of 6 mm after 15 minutes means 24 mm per hour.