Flow sets the zone size, and everything else follows
An irrigation zone is a group of heads on one valve, and the only hard limit on how many heads it can hold is how much water your supply can deliver at the pressure the heads need. Add heads beyond that and you do not get a slightly weaker system — you get every head in the zone throwing short at once, which pulls the overlap apart and leaves dry ground between them. The symptom is a lawn with rings or corners that never green up, and it is almost always a design fault rather than a controller setting.
So measure the flow first, before anything else. Open an outside tap fully, time how long it takes to fill a five gallon bucket, and divide 300 by the seconds to get GPM. Twenty seconds is 15 GPM. Then design to about 75 to 85 percent of that, because pipe friction, fittings and elevation all take their cut between the meter and the nozzle, and because someone will run a shower while the system is on.
Head spacing is head-to-head, not edge-to-edge
A sprinkler does not lay water down evenly across its circle. It puts down most near the head and thins towards the outside of the throw, which is why the industry standard is head-to-head spacing: each head is placed so its stream reaches the next head, and every point on the ground is covered by at least two overlapping patterns. Set them at the edge of their radius and the ground midway between them gets a fraction of the water the ground near a head gets.
| Head type | Typical operating pressure | Typical radius | Rough flow |
|---|---|---|---|
| Fixed spray | 30 psi | 12-15 ft | 1-2.5 GPM |
| Rotating nozzle (MP style) | 40 psi | 15-30 ft | 0.3-1.5 GPM |
| Gear rotor | 45 psi | 25-35 ft | 1.5-6 GPM |
Every one of those figures is a rough orientation only. Real values come from the manufacturer's nozzle chart for the specific nozzle, read at the pressure that will actually exist at the head — and a rotor at 35 psi throws meaningfully shorter and flows less than the same rotor at 50. Triangular spacing covers about 15 percent more ground per head than square spacing for the same radius, which is worth having on open lawn; square is easier to lay out and fits rectangular areas with less waste at the edges.
Precipitation rate and run time
Everything about scheduling comes from one conversion: an inch of water over a square foot is 0.623 gallons. That falls straight out of the units — a twelfth of a cubic foot, times 7.48 gallons to the cubic foot.
From there, a zone's precipitation rate in inches per hour is 96.3 times the zone GPM divided by the area it covers in square feet. (The 96.3 is just 60 minutes divided by 0.623.) A head flowing 2.5 GPM over 780 square feet of its own patch works out at about 0.49 in/hr, so half an inch of water takes about an hour. To get run time directly: minutes equals the target depth in inches times 0.623 times the area in square feet, divided by the zone's total GPM.
Worth checking against reality, because catch cans do not lie: put half a dozen straight-sided containers around a zone, run it for fifteen minutes, measure the depth in each with a ruler, and multiply by four. That gives you both the actual rate and — more usefully — how uneven it is. If one can has twice what another has, no run time will water that zone correctly and the layout needs fixing.
When the rate is too fast for the soil
Soil takes water at its own pace. A sandy loam might accept two inches an hour; a compacted clay might manage a fifth of that. Apply faster than the soil accepts and the surplus runs downhill, taking whatever was on the surface with it. High precipitation rates on slopes are where this bites hardest.
The fix is cycle-and-soak: split the run time into two or three shorter cycles with twenty or thirty minutes between them, letting each application soak in before the next. The total depth applied is the same; the amount that stays where you put it is much higher. Nearly every modern controller supports this, and on clay or any noticeable slope it is not optional.
What this does not cover
Pipe sizing and friction loss, valve and backflow selection, wire runs, and the pressure that actually arrives at the far head are all real design work and none of it happens here. This calculator assumes the heads get their rated pressure; whether they do depends on pipe diameter, length, fittings and elevation change across the run, and undersized pipe is the most common reason a zone that looks fine on paper performs badly on the ground. Backflow prevention on a potable supply is a plumbing-code matter and in most jurisdictions is not a homeowner's choice. If the system connects to drinking water, that part belongs with someone licensed to do it.
Questions people ask
How many sprinkler heads can I put on one zone?
Divide your usable flow by the flow of one head. If you measured 12 GPM and design to 80 percent of it, you have 9.6 GPM to spend; gear rotors at 2.5 GPM each give you three heads, low-flow rotating nozzles at 0.7 GPM give you thirteen. That is the whole calculation, and it is why nozzle choice changes a system design more than anything else. Do not fill a zone right to the limit — leave a head of margin, because nozzle flows are nominal and the pressure at the far end of a run is always lower than at the valve.
How long should I run my sprinklers?
Long enough to apply the depth you want, which depends on the zone precipitation rate rather than on any general rule. Work out the rate as 96.3 times the zone GPM divided by the square feet it covers, then divide your target depth by that rate. A zone running at 0.5 in/hr needs an hour for half an inch; a spray zone at 1.6 in/hr needs about nineteen minutes for the same depth, which is why spray zones and rotor zones should never share a run time. Verify with catch cans rather than trusting the arithmetic — they measure what actually lands on the grass.
How much water does an inch of irrigation take?
0.623 gallons per square foot, which is a twelfth of a cubic foot times 7.48 gallons per cubic foot. Over 1,000 square feet that is 623 gallons; over a 5,000 square foot lawn, 3,115 gallons for a single inch. Those numbers are worth knowing before you set a schedule, because a lawn wanting an inch and a half a week through a dry summer is a genuinely large volume of water and it shows up on the bill. A 10 GPM zone covering 1,000 square feet takes about 62 minutes to put down an inch.
What is head-to-head spacing and do I need it?
It means placing heads so that the spray from each one reaches the next head — spacing equal to the throw radius, not double it. You need it because sprinklers do not distribute evenly across their circle: output is concentrated near the head and tails off towards the edge of the throw, so the design assumes patterns overlap and the water at any point comes from two or more heads. Spacing at anything much beyond the radius produces dry ground between heads that no amount of run time fixes. In windy sites, tighten to 85 or 90 percent of the radius rather than relaxing it.
Why do I have dry spots even though the sprinklers run for ages?
Almost always coverage rather than duration, and running longer makes the wet parts wetter without fixing the dry ones. Work through it in order: heads spaced further apart than their throw radius; nozzles too small for the spacing or a mix of nozzle types on one zone; pressure at the head below what the nozzle needs, which shortens the throw; too many heads on the zone for the available flow; heads sunk below grade or blocked by grass; or arcs misadjusted so an area is only being hit by one head instead of two. Catch cans around the zone will tell you which within a quarter of an hour.