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Calculators / Water and power

How many sprinkler zones do you need?

Test your pressure and flow at the hose bib, then measure the lawn. You'll get how many heads each zone can run and how many zones it takes.

These are example numbers. Put in yours.

Your lawn

Shape of the lawn
ft
ft

The heads

Head type
ft

Head to head. Sprays 10 to 15 ft, rotary 13 to 30 ft, rotors 25 to 40 ft.

Your water

psi

Gauge on the hose bib nearest the meter, nothing else running.

gpm

Time a 5 gallon bucket at full blast. 300 ÷ seconds = gpm.

Plan this many zones

4 zones

12 spray heads, 3 per zone. Get a controller with at least 4 stations, more if you'll add beds.

Heads
12spray heads at 15 ft
Per zone
36 gpm
Usable flow
7.5 gpm
Pressure
55 psi
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area = 40 ft × 30 ft = 1200 sq ft

heads = (40 ÷ 15, rounded up, + 1) × (30 ÷ 15, rounded up, + 1) = 12

usable flow = 10 gpm × 75% = 7.5 gpm

heads per zone = 7.5 ÷ 2 gpm = 3, rounded down

zones = 12 ÷ 3 = 4, rounded up

Misting

Sprays work best at 30 psi. Buy pressure-regulated bodies (PRS) so they don't fog at your pressure.

One type per zone

Keep sprays, rotors and rotary nozzles on their own zones. They put down water at different rates.

Backflow

Most towns require a backflow preventer on the sprinkler line, and some want a plumber to test it. Check before you dig.

Rain sensor

A rain sensor shuts the system off when it rains. Some towns require one. It pays for itself in water.

See rain sensors on Amazon

How we work it out

Put a pressure gauge on the hose bib nearest the meter, with nothing else running. That's your static pressure. Then time how long a 5 gallon bucket takes to fill at full blast.

flow (gpm) = 300 ÷ seconds to fill 5 gallons

We design on 75% of that flow, so the zone still runs when someone uses water in the house.

heads per zone = flow x 0.75 ÷ gpm per head, rounded down

zones = total heads ÷ heads per zone, rounded up

We count heads on a square grid, head to head, edges and corners included. We use an average gpm per head: 2 for sprays, 0.8 for rotary nozzles and 3 for rotors. Your nozzle chart has the exact number.

Sprays need about 25 psi static, rotors and rotary nozzles about 30. You lose 10 to 15 psi in the meter, backflow and pipe. Over 80 psi, the house needs a pressure-reducing valve.

Where the numbers come from

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