Sprinkler Zone & Pipe Size Planner
How many heads fit on a zone, what pipe carries them, and why a rotor in a corner waters four times as fast as one in the open.
What each arc actually puts down
Pipe capacity and friction at this flow
Capacity is 5 ft/s times the bore area — not a rating on the pipe. Friction is Hazen-Williams at C=150.
A rotor in a corner waters four times as fast
On a rotor the arc is set by the gearbox and the flow by the nozzle, and the two are independent. Turn a rotor down to a quarter circle and it keeps its full 2.6 gallons a minute but now delivers it into a quarter of the ground — 1.11 inches an hour against 0.28. Exactly four times. Spray heads escape it completely, because on a spray the nozzle is the arc: the quarter, half and full nozzles flow 0.92, 1.85 and 3.70 gpm, which is 1 : 2 : 4, so every arc waters at the same rate. Rotors have to be matched by hand, by fitting a smaller nozzle number to the smaller arcs — and almost nobody does. It's the commonest reason a professionally installed system still waters unevenly.
Pipe size is a velocity limit, not a pressure limit. The familiar capacities — 4–5 gpm through half-inch, 8 through three-quarter, 13 through one inch, 22 through inch-and-a-quarter — are simply five feet per second times the bore area, rounded down. So you can't fix an overloaded zone by turning the pressure up: more pressure gives more flow, which gives more velocity, which gives water hammer — and water hammer breaks fittings rather than degrading gracefully. You fix an overloaded zone by splitting it.
One pipe size up cuts friction loss to about a third. Hazen-Williams puts loss on diameter to the power of 4.87, so stepping from three-quarter inch to one inch — a 25% bigger bore — divides the loss by 2.95. The flow exponent is 1.852, which means doubling the flow multiplies loss by 3.6: not two, and not four. Upsizing the mainline is nearly always cheaper than the extra zone and valve the alternative costs.
- Measure the supply, don't estimate it. Time a five-gallon bucket off an outside tap with everything else off — gpm is 300 divided by the seconds. Then read static pressure off a gauge on the same tap. Designing from the pipe size at the road is how zones end up two heads over.
- Design to about 75% of what you measured. The rest is margin for somebody running a tap while the system is on, and for the meter reading generously.
- Space head-to-head — each head throwing as far as the next, not to the midpoint. It looks like heavy overlap and isn't: a sprinkler applies far more close in than at the edge of its throw. Spacing at 60% of diameter is the commonest layout error, and it shows as dry rings exactly halfway between heads.
- Overloading fails everywhere at once. Pressure drops at every head together, so the whole zone throws short and all the gaps open simultaneously — which is why it reads as "the system is weak" rather than "that zone has too many heads".
- One head type per zone. A spray at 1.58 in/hr on the same valve as a rotor at 0.28 means no run time is right for either.
How to use
- Measure the supply with a bucket and a pressure gauge before anything else.
- Pick the head type and enter how many of each arc the zone needs.
- Check the velocity in the pipe, not just whether the flow fits.
- Match part-circle nozzles by hand if the head type is a rotor or an impact.
Frequently asked questions
How many sprinkler heads can I put on one zone?
Divide about three quarters of your measured flow by the flow of one head. A 10 gpm supply gives 7.5 gpm to design with, which is two full-circle spray heads at 3.7 gpm each, or two rotors at 2.6, or seven rotary nozzles. The margin matters — the budget is what is actually there when somebody also runs a tap indoors.
Why does a rotor in a corner water faster than one in the open?
Because on a rotor the arc is set by the gearbox and the flow by the nozzle, and the two are independent. Turn it down to a quarter circle and it keeps its full flow but delivers it into a quarter of the ground — exactly four times the application rate. Fitting a smaller nozzle number in the part-circle heads is what matches them, and it is the step almost everybody skips.
Do spray heads have the same problem as rotors?
No, and it is worth knowing why. On a spray head the nozzle is the arc, so the quarter, half and full nozzles flow 0.92, 1.85 and 3.70 gpm — a ratio of 1 : 2 : 4 that exactly cancels the difference in area covered. Every arc waters at the same rate and mixing them on a zone is safe. Rotary nozzles are matched the same way, deliberately.
What size pipe do I need for a sprinkler zone?
Whatever keeps the water under five feet per second. That works out at roughly 4 to 5 gpm through half-inch, 8 through three-quarter, 13 through one inch and 22 through inch-and-a-quarter — figures that are simply five feet per second times the bore area, rounded down. They are a velocity limit, not a pressure rating on the pipe.
Why is there a velocity limit on irrigation pipe?
Water hammer. When a valve closes, the pressure spike is proportional to how fast the water was moving, and above about five feet per second it becomes enough to crack fittings and split pipe over time. It is not a gradual loss of performance — the system works fine until something breaks underground, which is the worst possible failure mode to design towards.
Can I fix an overloaded zone by increasing the pressure?
No, and trying makes it worse. More pressure produces more flow, more flow means higher velocity in the same pipe, and higher velocity is exactly the thing the limit exists to prevent. An overloaded zone is fixed by splitting it into two zones, by fitting lower-flow nozzles, or by running a larger pipe — never by turning something up.
How much pressure do I lose in the pipe?
It follows Hazen-Williams: loss goes as flow to the power 1.852 over diameter to the power 4.87. One inch Schedule 40 at 10 gpm loses about 6 feet of head — 2.6 psi — per hundred feet. The diameter exponent is the striking part: one pipe size up cuts the loss to roughly a third at the same flow, which usually costs less than the extra zone and valve it saves.
Does doubling the flow double the friction loss?
No, it multiplies it by about 3.6, because the flow exponent is 1.852 rather than 1. That is why an overloaded zone loses pressure faster than people expect, and why halving the flow through a run recovers far more than half the loss — it leaves about 28 per cent of it, not 50.
How do I measure my available flow and pressure?
Turn everything else off, time how long an outside tap takes to fill a five-gallon bucket, and divide 300 by the seconds to get gallons per minute. Then screw a pressure gauge onto the same tap and read it with nothing running for the static pressure. Both take five minutes, and designing without them is how zones end up two heads over.
How far apart should sprinkler heads be?
Head to head — each one throwing far enough to reach the next, not to the midpoint between them. It looks like far too much overlap and it is not: a sprinkler applies much more water close in than at the edge of its throw, so the overlap is what makes coverage even. Spacing at 60 per cent of the diameter is the commonest layout error and it shows as dry rings exactly halfway between heads.
What happens if a zone has too many heads?
Pressure falls at every head simultaneously, so the whole zone throws short and every gap opens at once. That is why it reads as "the system is weak" rather than as "that zone has one head too many" — nothing fails in a way that points at the cause, and turning the run time up does not help, because the dry areas are not being reached at all.
Can I mix spray heads and rotors on one zone?
Not usefully. A 15 ft spray applies about 1.58 inches an hour and a 30 ft rotor about 0.28 — a factor of nearly six. Whatever run time the controller uses, one part of the lawn gets several times what the other does. It is a plumbing decision rather than a programming one, and no amount of adjusting the schedule corrects it.
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