Sprinkler Coverage Calculator

Counting heads

The rule every irrigation designer works to is head to head: each sprinkler must throw far enough to reach the next one. A single head waters its own circle weakly at the edge and strongly near the middle, and the only way to even that out is to let neighbouring circles overlap all the way to each other’s centres. So the grid spacing equals the spray radius, not the diameter.

That gives the count:

Heads across = (Lawn length ÷ Spacing) rounded down, + 1 Heads down = (Lawn width ÷ Spacing) rounded down, + 1

The + 1 is the part people leave out, and it is why a bare area ÷ spacing² estimate comes up short every time. A 45 ft square lawn with 25 ft rotors is not 2 heads. You need a head at each end of every row, so it is 2 × 2 = 4, and the difference only grows on awkward narrow strips.

Flow, precipitation rate, and run time

Precipitation rate is how fast the sprinkler puts water down, in inches per hour:

Precipitation rate = (Gallons per minute × 96.25) ÷ Area covered by one head (sq ft)

96.25 falls out of the arithmetic: one inch of water over one square foot is 0.623 gallons, and there are 60 minutes in an hour, so 60 ÷ 0.623 = 96.25.

Worked example. A rotor on a 25 ft head-to-head grid covers a 25 × 25 = 625 sq ft square, and delivers 3 gallons per minute.

• Precipitation rate = (3 × 96.25) ÷ 625 = 0.46 inches per hour
• One inch of water therefore takes 60 ÷ 0.46 = 130 minutes, split across the week

That is the number worth knowing before anything is glued together. Rotors are slow, which is why rotor zones run for half an hour and spray zones run for ten minutes.

Never mix the two families in one zone. Put a spray head and a rotor on the same valve and the spray delivers three times as much water in the same run. One patch drowns, the other stays dry, and no amount of controller tweaking fixes it because they share a timer. Split them onto separate zones.

Anything above about 1.5 inches per hour will outrun heavy clay or any slope, and the surplus runs into the street. Two short cycles half an hour apart put the same water down and all of it soaks in.