Guides

Sizing a sprinkler zone

The question people ask is how many heads a zone can have. The honest answer is that a zone is limited by the water coming out of your tap, not by the size of your yard — and the second constraint, spacing, is what decides whether the grass is actually green between the heads. Both are arithmetic you can do in an afternoon.

Start with the water you actually have

Every zone decision follows from one number: how many gallons per minute your supply can deliver. Measure it rather than guessing. Put a 5 gallon bucket under an outside spigot, open it all the way, and time the fill:

Thirty seconds means 10 GPM. Twenty seconds means 15. Do it with nothing else in the house running, and do it at the time of day you would actually water, because pressure sags when the neighbourhood is using it.

Then do not design to that number. Take 75% of it as your working capacity:

The margin covers friction loss down the pipe, elevation, and the fact that your measured flow was at one spigot on one afternoon. A typical three-quarter inch residential meter supplies somewhere around 18 to 20 GPM at 40 to 60 PSI, which is why so many residential zones land at 15 GPM or less. Design past your capacity and you do not get a warning — you get the far heads on every zone quietly under-throwing, and dry rings that look like a lawn disease.

Add up heads, not area

A zone is full when the heads on it add up to your design GPM. That is the whole rule:

Each head's flow comes off the manufacturer's nozzle chart, and it depends on the nozzle, the arc and the pressure — not on the head's brand or price. Look yours up rather than using a rule of thumb, because the spread is large. Rotors commonly run anywhere from well under 1 GPM to 8 GPM depending on nozzle and arc.

Arc matters as much as nozzle. On a properly matched rotor set, a half-circle head uses half what a full-circle head uses and a quarter-circle a quarter — 2 GPM at 90°, 4 GPM at 180°, 8 GPM at 360° is the classic matched set. That is deliberate: it is what makes every head in the zone lay down water at the same rate.

So with 15 GPM of design capacity you might fit seven or eight 2 GPM heads, or three 4 GPM ones. Count the gallons, not the heads.

Spacing: head to head, always

A sprinkler does not water its circle evenly. Output is heaviest near the head and tapers off toward the edge of the throw, so if you space heads so their patterns just touch, the ground between them gets a fraction of what the ground beside each head gets. The fix is head-to-head coverage: every head throws far enough to reach the next one.

A head with a 15 foot radius goes 15 feet from the next head, not 30. It feels like overkill and it is not — the overlapping thin outer edges are what add up to even coverage. Three practical limits sit on top of that rule:

• Where an odd shape forces a wider gap, cap it at 0.6 × the diameter. For 15 foot radius heads that is 18 feet, and it should be the exception rather than the layout.
• Wind above about 10 mph means going closer than head to head — around 45% of the diameter — because the throw on the downwind side collapses.
• For rotors, spacing in feet should not exceed the operating pressure in PSI at the head. At 30 PSI, 30 feet is your ceiling regardless of what the catalogue radius says.

Staggering the heads in a triangular grid rather than a square one covers more ground per head, so if you are trying to reduce head count without opening the spacing, that is the lever to pull.

Never mix sprays and rotors on one valve

This is the mistake that produces a lawn with both drowned patches and dead patches on the same zone. Fixed spray heads apply water roughly three times as fast as rotors — on the order of 1.5 inches per hour against about 0.5 — because a spray empties its whole pattern continuously while a rotor sweeps a stream across it.

Put both on one valve and there is no run time that works. Water long enough for the rotors and the spray areas flood and run off. Water short enough for the sprays and the rotor areas never get enough. Group sprays with sprays, rotors with rotors, and put drip on a zone of its own.

The underlying idea is matched precipitation: every head on a zone should apply water at the same depth per hour, so one run time suits all of it. If you want to check yours:

Split zones by what the plants want, too

Even with matched heads, one zone should cover ground with one watering need. Sun and deep shade dry out at completely different rates; lawn and established shrubs want different depths and frequencies; a south-facing slope sheds water that a flat bed would hold. Every boundary like that is a reason for a separate valve, and it costs far less to plan now than to retrofit.

Putting it together

1. Measure your flow with the bucket test, then multiply by 0.75.
2. Pick a head type for each area — rotors for open lawn, sprays for tight or narrow ground.
3. Lay heads out at head-to-head spacing, corners first, then edges, then fill the middle.
4. Read each head's GPM off the nozzle chart for its arc.
5. Group heads into zones until each zone's total reaches your design GPM, never mixing types.
6. Split further wherever sun, shade or plant type changes.

Plan your heads in the planner

Sprinkler heads drop onto the sketch as individual items, so you can place them at head-to-head spacing over a to-scale drawing of your yard and count what you actually need before buying anything. Draw the lawn first, get its real dimensions, and the spacing grid follows from there.