Sprinkler Calculator

Sprinkler formula

Heads = (Length / spacing, rounded up, + 1) x (Width / spacing, rounded up, + 1), spacing = head radius
Zones = Heads x gpm per head / available gpm, rounded up
Flow per zone = Heads per zone x gpm per head
Minutes per week for 1 in = 60 / precipitation rate (rotor 0.5, spray 1.5, rotary nozzle 0.4 in per hour)

Worked example

A 90 x 50 ft front lawn with 15 ft spray heads on a 12 gpm supply: 90 / 15 = 6, plus 1, is 7 heads along the length; 50 / 15 = 3.33, rounded up to 4, plus 1, is 5 across the width. 7 x 5 = 35 heads. At 1.5 gpm each they draw 52.5 gpm, so 52.5 / 12 = 4.4, rounded up to 5 zones of 7 heads at 10.5 gpm each.

Spray heads put down about 1.5 in per hour, so each zone runs 60 / 1.5 = 40 minutes a week, best split into two 20 minute waterings. At 10.5 gpm per zone a 1 in main is enough.

Quick reference

Counting heads and zones

Start by measuring the flow you actually have: put a 5 gallon bucket under the hose bib, open it fully and time the fill. Five gallons in 30 seconds is 10 gpm. That number caps how many heads can run at once, and every zone must stay under it, ideally with 10 to 20 percent to spare so pressure holds up at the far head.

Pick one head type per zone and space the heads at the throw radius so the spray from each head reaches its neighbors. A rectangle gets a head at every corner, along every edge and across the middle in a grid; irregular lawns need a scale drawing, but the grid count is a good first estimate. Multiply the heads by their flow, divide by your available flow and round up for the number of zones.

Run times and pipe

Rotors and rotary nozzles apply water slowly, around half an inch an hour, so a zone runs 2 hours a week to deliver 1 inch; spray heads put down 1.5 inches an hour and need only 40 minutes. Split the weekly total into 2 or 3 waterings early in the morning, and cut back in spring and fall. A rain gauge or a few tuna cans on the lawn will show whether the real rate matches these figures.

Run a 1 in poly or PVC main line from the backflow preventer to the valve manifold for systems in the 10 to 15 gpm range, then 3/4 in or 1 in laterals to the heads. Most areas require a backflow preventer on any irrigation tap and a permit for the plumbing connection, so check with the local water utility before you dig.

Key facts

• Sprinkler heads are spaced head to head: each head throws far enough to wet the next head, which is the only way to get even coverage.
• Rotors apply about 0.5 in per hour, rotary nozzles about 0.4 in per hour, and fixed spray heads 1.5 in per hour or more.
• Most lawns need 1 to 1.5 in of water a week in summer, applied in 2 or 3 deep waterings rather than daily sprinkles.
• A 3/4 in hose bib on a 40 to 60 psi supply usually delivers 8 to 12 gpm, and spray heads and rotors should never be mixed on one zone.
• Pipe is sized to keep water under 5 ft per second: 3/4 in for up to 8 gpm, 1 in for up to about 13 to 15 gpm, 1-1/4 in for up to 22 gpm.

Frequently asked questions

Sources and references

• •5000 / 5000+ Series rotor product page (25 to 50 ft radius, 0.76 to 9.63 gpm) (Rain Bird)
• •1800 Series spray sprinkler bodies product page (2.5 to 15 ft spray, 8 to 24 ft rotary nozzle radius) (Rain Bird)
• •Landscape Irrigation Design Manual (head spacing and precipitation rate calculations) (Rain Bird)
• •Methods to Schedule Home Lawn Irrigation (inches per week, spray vs rotor precipitation rates, catch-can test) (Colorado State University Extension)