How many sprinkler heads can go on one zone?
As many as the tap can feed at the pressure the nozzles need. Every head has a flow — printed on the nozzle chart in gallons per minute at a given pressure — and the zone's flow is the sum of them. The supply is what a bucket test measures. Divide one by the other and you have the head count; the rest of this page is the two things that division leaves out, which are the arc of each head and the pipe in between.
A 5 gallon bucket that fills in 30 seconds is 10 GPM. That is the tap wide open with nothing pushing back; a spray head needs 30 psi at the nozzle, and pressure and flow trade against each other, so a zone can't have the whole 10. Designing to about three-quarters of it — 7.5 GPM — is the standard margin, and 7.5 ÷ 3.7 GPM is 2 full-circle 15 ft sprays. Or 4 half-circles, which is why the calculator asks for the arcs.
The bucket test, and what it's worth in heads
Wide open at the hose bib nearest the meter, nothing else in the house running, time a 5 gallon bucket. 300 ÷ seconds is GPM. A pressure gauge on the same bib, with everything off, gives static pressure — the second number the pipe section needs.
| 5 gal fills in | Tap GPM | Design GPM | 15 ft sprays, full | MP2000 halves | Rotors, #3 nozzle |
|---|---|---|---|---|---|
| 15 s | 20.0 | 15.0 | 4 | 17 | 5 |
| 20 s | 15.0 | 11.3 | 3 | 12 | 3 |
| 30 s | 10.0 | 7.5 | 2 | 8 | 2 |
| 45 s | 6.7 | 5.0 | 1 | 5 | 1 |
| 60 s | 5.0 | 3.8 | 1 | 4 | 1 |
Design GPM is 75% of the bucket figure. Heads per zone at 3.7 GPM (15 ft spray, full circle), 0.87 GPM (MP2000-class rotary nozzle, half circle) and 3 GPM (rotor, #3 nozzle). Whole heads only.
A tap that fills the bucket in a minute — 5 GPM — is a real constraint: two full-circle sprays per zone. That is a well, a long ½ in service line, or a hose bib on the far side of the house from the meter. Test at the closest bib you have, and if the number is low, it's the supply that needs attention before any zone does.
GPM per head: the arc matters — unless it's a rotor
A spray or rotary nozzle set to a quarter circle passes a quarter of the water, because the nozzle is physically a quarter of the orifice. That's what keeps the precipitation rate even across arcs — "matched precipitation" — and it means a zone of corner and edge heads draws far less than the same count of full circles. A rotor is different: it flows its whole nozzle over whatever arc you set, so a half-circle rotor puts down twice the rate of the full-circle one beside it unless you fit it a half-flow nozzle.
| Nozzle | psi | Radius | Full | Half | Quarter | Rate, full circle |
|---|---|---|---|---|---|---|
| Spray, 15 ft nozzle | 30 | 15 ft | 3.7 | 1.85 | 0.92 | 1.58 in/hr |
| Spray, 12 ft nozzle | 30 | 12 ft | 2.6 | 1.3 | 0.65 | 1.74 in/hr |
| Spray, 10 ft nozzle | 30 | 10 ft | 1.58 | 0.79 | 0.39 | 1.52 in/hr |
| Spray, 8 ft nozzle | 30 | 8 ft | 1.05 | 0.52 | 0.26 | 1.58 in/hr |
| Rotary nozzle, MP1000 class (8–15 ft) | 40 | 14 ft | 0.87 | 0.43 | 0.22 | 0.43 in/hr |
| Rotary nozzle, MP2000 class (13–21 ft) | 40 | 19 ft | 1.74 | 0.87 | 0.44 | 0.46 in/hr |
| Rotary nozzle, MP3000 class (22–30 ft) | 40 | 28 ft | 3.61 | 1.8 | 0.9 | 0.44 in/hr |
| Rotor, #2 nozzle | 45 | 30 ft | 2 | 2 | 2 | 0.21 in/hr |
| Rotor, #3 nozzle | 45 | 35 ft | 3 | 3 | 3 | 0.24 in/hr |
| Rotor, #4 nozzle | 45 | 40 ft | 4 | 4 | 4 | 0.24 in/hr |
GPM. Sprays: Rain Bird 1800 MPR nozzles at 30 psi. Rotary: Hunter MP Rotator classes at 40 psi. Rotors: the number on a residential rotor nozzle is roughly its GPM near 45 psi (Rain Bird 5000, Hunter PGP); a rotor's radius depends on the nozzle and pressure, so read its chart. Rate is 96.25 × GPM ÷ radius² at head-to-head square spacing.
A 15 ft spray head puts down 1.58 in/hr; an MP2000-class rotary nozzle at its own spacing puts down 0.46. Same zone, same timer, and the spray side gets 3.4× the water — the run time that soaks one drowns or starves the other. Zone by head type, always. Then the run time is whatever the run-time calculator says for that zone's rate, and the button above hands the rate across.
What size pipe for a sprinkler zone?
The one that keeps the water under 5 ft/s. Faster than that, friction climbs steeply (loss goes with flow to the 1.85 power) and, worse, the valve slamming shut on a fast-moving column of water is what hammers pipe and fittings apart. Speed is 0.4085 × GPM ÷ bore², and the bore is the inside diameter — which is why Class 200 carries more than Schedule 40 at the same nominal size: the wall is thinner.
| Nominal size | Class 200 PVC | Schedule 40 PVC | Polyethylene (poly) |
|---|---|---|---|
| ¾ in | 10.6 GPM | 8.3 GPM | 8.3 GPM |
| 1 in | 17.3 GPM | 13.5 GPM | 13.5 GPM |
| 1¼ in | 27.6 GPM | 23.3 GPM | 23.3 GPM |
| 1½ in | 36.2 GPM | 31.7 GPM | 31.7 GPM |
| 2 in | 56.5 GPM | 52.3 GPM | — |
Maximum flow at 5 ft/s, from the inside diameters (ASTM D1785 for Schedule 40, D2241 for Class 200; SIDR poly shares Schedule 40's bores). These reproduce the Rain Bird lateral chart: ¾ in Class 200 tops out at 10 GPM, 1 in at 17.
In practice that makes the choice simple. Most spray zones on a residential tap run 6–10 GPM and are fine in ¾ in Class 200 or 1 in Schedule 40; rotor zones run 10–17 and want 1 in; anything past 17 GPM is 1¼ in, and probably two zones.
Friction loss: what a long run costs you
Pipe isn't free to push water through. The loss over a lateral is the chart figure per 100 ft, times the length, plus a tenth for the fittings — and it comes straight off the pressure the last head sees.
| GPM | ¾ in | 1 in | 1¼ in | 1½ in | 2 in |
|---|---|---|---|---|---|
| 4 | 1.41 | 0.44 | 0.11 | 0.05 | 0.02 |
| 6 | 2.99 | 0.92 | 0.24 | 0.11 | 0.03 |
| 8 | 5.09 | 1.57 | 0.41 | 0.20 | 0.06 |
| 10 | 7.69 | 2.38 | 0.63 | 0.30 | 0.09 |
| 12 | 10.78 | 3.33 | 0.88 | 0.41 | 0.12 |
| 15 | 16.30 | 5.04 | 1.33 | 0.63 | 0.19 |
| 20 | 27.77 | 8.58 | 2.26 | 1.07 | 0.32 |
| 25 | 41.98 | 12.97 | 3.42 | 1.61 | 0.48 |
| 30 | 58.84 | 18.18 | 4.79 | 2.26 | 0.67 |
psi lost per 100 ft, Schedule 40 PVC (C = 150), Hazen-Williams. Shaded cells are over 5 ft/s — the pipe is too small there regardless of the loss. Class 200 loses less at every size; poly (C = 140) about 14% more than PVC of the same bore.
psi per foot = 4.52 × GPM1.852 ÷ (C1.852 × bore4.8655). Every manufacturer's friction chart is that formula printed out; C is the pipe's smoothness (150 for PVC, 140 for poly). Note the bore exponent: one nominal size up cuts the loss by two-thirds or more. The calculator applies the full zone flow to the whole run, which overstates the loss — the flow drops as heads branch off, and a designer sizes each segment for what actually passes through it. Treat the figure as a ceiling; if the zone passes with it, it passes.
Eight GPM through 150 ft of ¾ in Schedule 40 loses 8.4 psi (at 4.8 ft/s, right at the limit). The same run in 1 in loses 2.6 psi at 3.0 ft/s. The difference in pipe cost is a few dollars per stick, and the 5.8 psi it buys is the difference between the far heads throwing their rated radius and misting short. When a zone is marginal, upsize the pipe before you drop heads.
Will the last head still have pressure?
Start with the static pressure at the bib. Subtract the friction over the run, 0.433 psi for every foot the heads sit above the valve (add it back going downhill), and about 10 psi for the backflow preventer and zone valve together. What's left is what the nozzle gets, and it needs roughly 30 psi for sprays, 40 for rotary nozzles, 45 for rotors. A 55 psi house with a 150 ft ¾ in run and a small rise often lands in the mid-30s — fine for sprays, short for the rotary nozzles it was designed around. That's the check the calculator's last panel does.
If you're short, the levers in order: upsize the pipe (friction is the only line you control), split the zone (less flow, less friction), fit pressure-regulating heads only if the problem is too much pressure, and — for a genuinely weak supply — a booster pump, which is a real fix and not a cheap one.
Why is the last head on my zone weak?
Read it from where the weakness is, not from the timer.
| What you see | Cause | Fix |
|---|---|---|
| Far heads throw short or mist; near heads are fine | Friction — too much flow for the pipe, or too many heads | Upsize the lateral or split the zone. Check the numbers above. |
| Every head on the zone is weak, including the first | Supply: low static pressure, a valve's flow control turned down, a clogged filter or a backflow preventer starting to fail | Gauge at the bib; open the valve's flow-control stem; service the backflow. |
| Heads on the uphill side are weak | Elevation: 0.433 psi per foot of rise | Zone the slope separately; upsize the pipe to claw back friction. |
| Fine fog everywhere; wet windows across the street | Too much pressure — sprays above 30 psi atomize | Pressure-regulating spray bodies (30 psi), or a regulator at the valve. |
| Rotors stall or turn slowly | Below the rotor's operating pressure | Fewer heads on the zone, bigger pipe, or a rotor rated for low pressure. |
| Dry crescent between two rotors, one of them a half circle | Arc mismatch: the half circle puts down twice the rate | Matched-precipitation nozzle set — halves get the half-flow nozzle. |
| A zone that used to be fine is suddenly weak | A broken head, a cracked lateral, or a nozzle full of grit | Walk it while it runs; look for the geyser or the puddle. |
A zone that only fails in summer evenings is sharing supply with the house — showers, dishwasher — and wants a different start time, not a redesign.
Sources: Rain Bird, Landscape Irrigation Design Manual (Hazen-Williams form and C values, the 5 ft/s rule, pipe dimensions, pressure-loss budgeting); Hunter Industries, Residential Sprinkler System Design Handbook (bucket test, design capacity); Rain Bird 1800-series MPR and Hunter MP Rotator nozzle performance charts; ASTM D1785 and D2241 pipe dimensions; Irrigation Association, Landscape Irrigation Best Management Practices.