Head-to-head spacing
For even watering, each sprinkler should throw water all the way to the next head. So the spacing between heads equals the throw radius. Spacing them further apart leaves dry patches in the middle.
Heads = (length ÷ radius + 1) × (width ÷ row spacing + 1)
A lawn 40 ft by 30 ft with spray heads throwing 12 ft: 5 heads along the length × 4 rows = 20 heads.
Planning the rest of the job? Use the Irrigation water calculator and the Lawn area calculator too.
What types of sprinkler are there?
Spray heads
Fixed fan of water, 8 to 15 ft (2.5 to 4.5 m). About 1.5 gpm each.
Best for small and narrow lawns
Rotor heads
Rotating streams, 20 to 50 ft (6 to 15 m). About 3 gpm each.
Best for medium and large lawns
Impact sprinklers
Classic ticking heads, up to 40 ft (12 m) or more.
Best for big open areas
Why do zones matter?
Your water supply can only feed so many heads at once. Measure the flow at an outdoor tap by timing how long it takes to fill a 5 gallon bucket. Divide the total flow of all the heads by that figure to see how many zones to split the system into.