Sprinkler spacing and heads for your lawn

What the precipitation rate tells you

The precipitation rate is the depth of water the sprinklers apply per hour, in inches per hour or millimeters per hour. It sets the run time: a zone that applies half an inch an hour needs two hours for a full inch.

How far apart should sprinkler heads be?

The usual rule is head-to-head coverage: each head's spray reaches the next head, so the spacing equals the throw radius (half the diameter on the nozzle chart). Enter 100 percent of the radius for that. Going wider leaves dry spots between heads and going closer wastes water.

How do I calculate the precipitation rate?

For square spacing, rate in inches per hour = 96.3 × gallons per minute ÷ (S × S), with S the spacing in feet. For triangular spacing the area per head is S × L with L = 0.866 × S, so the rate is 96.3 × gpm ÷ (S × S × 0.866). In metric, rate in mm/h = 1,000 × cubic meters per hour ÷ area per head in m².

Is triangular spacing better than square?

The Michigan State turfgrass note says triangular spacing typically provides better coverage than square. The price is more heads for the same spacing, because the rows are only 0.866 of the spacing apart, and an offset layout that is a little harder to pipe. The calculator shows both layouts side by side.

How long should I run the sprinklers?

Run time in minutes = inches wanted × 60 ÷ precipitation rate. At 1.48 in/h, one inch takes 40.5 minutes. Water in cycles that soak in instead of running off, and measure the real rate with catch cans, because pressure, wind and worn nozzles change it.

Does the formula work for part-circle heads?

Rain Bird multiplies the flow by 360 and divides by the arc in degrees, so a half-circle head that puts out half the water has the same rate as a full-circle head. This page assumes full-circle heads, or part-circle heads whose nozzles are matched so the rate is the same; check the manufacturer's chart for the real flow.

Why is the constant 96.3 and not 96.25?

A gallon is 231 cubic inches, an hour is 60 minutes, and a square foot is 144 square inches, so 1 gallon per minute over 1 square foot is 231 × 60 ÷ 144 = 96.25 inches per hour. Rain Bird rounds this to 96.3, Toro uses 96.25. The difference is 0.05 percent.

How many gallons per minute can my zone supply?

Add up the flow of every head that runs at once. The calculator shows the total for the whole lawn, but a zone should hold only as many heads as your supply can feed at the pressure the nozzles need, so split a big lawn into several zones. Check the supply with the GPM calculator.

How the layout is counted

Heads sit on a grid. With head-to-head spacing the grid step is the radius. In a square layout the rows are S apart; in a triangular layout the rows are only 0.866 × S apart and every second row is shifted half a step. We count heads along the border too, so the edge of the lawn is watered by its own heads. This count is our own arithmetic, not a published formula: 20 heads for a 60 × 40 ft lawn with a 15 ft radius in a square layout against 23 triangular, because the triangular layout needs 5 rows instead of 4. A real design adjusts the grid to fit the lawn, so round the result and check it on a plan.

Worked example from the published formula

A Michigan State University turfgrass note compares a 20 gpm sprinkler at 65 ft spacing: 96.3 × 20 ÷ (65 × 65) = 0.456 in/h square, and 96.3 × 20 ÷ (65 × 65 × 0.866) = 0.526 in/h triangular. At 71 ft triangular it falls to 0.441 in/h, 16 percent less water for the same head. (The note prints .523 for the triangular 65 ft case; the formula gives 0.526.) Wider spacing means less water per hour, so run times must be longer.

Limits of the method

The formula divides the flow by the area each head serves, so it gives an average and says nothing about how even the watering is. The geometric limit, our derivation, is the square-root-of-two times the radius for a square layout and the square root of three times the radius for a triangular one, which is where the center of four or three heads is just at the edge of the spray; at that spacing there is no margin for wind or low pressure, and the calculator warns you. Wind, pressure above or below the nozzle chart and slopes all change the pattern.

Common mistakes

Using the diameter where the radius is needed, which gives a spacing twice as wide as it should be. Reading the flow at the wrong pressure from the nozzle chart. Mixing heads with different flows in one zone, which gives uneven rates. Applying the full-circle rate to a strip or corner head. Expecting a computed rate to replace a catch-can test.

Sources: Rain Bird, Precipitation Rate Equations and Rotor Nozzle Guide (rainbird.com/media/12082 and /media/2429; a manufacturer’s publication, so figures for its own heads); Toro, ResCom Catalog Formulas and Conversions (constant 96.25); Michigan State University, Turf Tips, December 7, 1989, the precipitation rate example. The layout count and the coverage limit are our own geometry. Related: lawn watering calculator, drip irrigation calculator, GPM calculator, plant spacing calculator. How we check our calculators.