How to design irrigation for a rectangular garden

Why rectangular gardens are trickier than they look

A rectangular garden seems like the simplest shape to water. In reality it hides specific traps: the corners are the farthest points from any perimeter sprinkler, and a rough design leaves them systematically dry.

The problem is geometric. A 13 ft (4 m) head set at 90° in a corner covers the left and right edges fine, but the diagonal toward the far corner of its quarter is 18.5 ft (5.6 m) long. If neighboring heads do not reach that point, you get an uncovered triangle.

The key is how a pop-up spreads water: output is highest near the nozzle and falls off toward the edge of the radius. That is what forces the head-to-head principle — every sprinkler must reach the position of the next one.

Measuring up: how to start right

Before choosing heads, measure the garden accurately. For a rectangle you only need length and width. Note where the water connection is (the garden spigot or tap-in point) and any obstacles: beds, trees, paths.

Also measure available pressure at the spigot with a gauge ($10–15). That number sets the maximum head radius you can use: at 30 PSI (2 bar) you can run 10–13 ft (3–4 m) heads; at 45 PSI (3 bar) you reach 16–20 ft (5–6 m).

The head-to-head rule on a rectangle

Head-to-head means every sprinkler reaches the position of the nearest one, so the maximum spacing equals the radius. (The overlap covers for the weak outer edge of each spray.)

For a 33×26 ft (10×8 m) garden the typical layout is 6 heads: one in each corner at a 90° arc, and one at the midpoint of each long side at 180°. The short 26 ft (8 m) sides, with 13 ft corner heads, are already covered without extra mid-side heads. A larger 39×33 ft (12×10 m) garden needs 8–9 heads, possibly including a 360° head in the middle if the open span exceeds twice the perimeter heads' radius.

Choosing arcs and radius

Most modern pop-ups have a continuously adjustable arc (say 40–360°), so one model covers corners, sides and center — you just dial in the angle. Radius depends on pressure and size: roughly 10–13 ft (3–4 m) heads up to about 540 sq ft (50 m²), 13–16 ft (4–5 m) up to 1,075 sq ft (100 m²), 16–20 ft (5–6 m) up to 2,150 sq ft (200 m²).

Rotary heads (rotors), which sweep a stream back and forth, beat fixed sprays for areas over about 320–430 sq ft (30–40 m²): they apply water more slowly, cutting runoff on clay and using less water per unit area.

Sizing the circuits

The total flow of a circuit's heads must stay under 80% of the available flow. With 5 GPM (20 L/min) available and 0.5 GPM heads, the maximum per circuit is about 8–9 heads.

For the 33×26 ft garden with 6 heads at 0.5 GPM, total demand is 2.8 GPM (10.8 L/min). With 4 GPM (15 L/min) available, all 6 fit on one circuit; with only 2.6 GPM (10 L/min), split into two circuits of 3 heads. Smaller circuits keep pressure steadier and coverage more even, so when in doubt prefer two circuits over one overloaded one.

A sample material list for a 33×26 ft garden

An 860 sq ft (80 m²) rectangle needs roughly: 6 adjustable-arc rotary pop-ups, about 100–115 ft (30–35 m) of 1-inch (25 mm) poly for the main line, 50–65 ft (15–20 m) of 1/2-inch (16 mm) poly for the branch runs, 6 tee fittings, 6 head outlets, one 1-inch solenoid valve and a 1–2 zone controller.

Rough materials cost: $160–380 depending on head quality — budget heads like the Hunter PGP or Rain Bird 5000 are reliable and plenty for home use. A DIY install is mostly the labor of cutting trenches about 8 inches (20 cm) deep. SprinklerMap generates the exact list from your own garden drawing, with pipe and fitting quantities calculated on the real runs.

Common questions

What about an L-shaped or irregular yard? Treat each rectangle separately and overlap the heads where the shapes meet — the same head-to-head rule applies to every edge. SprinklerMap handles irregular outlines directly, so you do not have to do the geometry by hand.

Do I really need a head in the exact corner? Yes. The corner is the farthest point from everything else, so a 90° head sitting right in it, aimed diagonally inward, is what keeps corners green. Skipping corner heads is the number-one reason rectangular lawns brown at the edges.

Spray or rotor on a small rectangle? Under about 320 sq ft (30 m²), fixed sprays are simpler and cover fine. Larger than that, or on clay and slopes, rotors apply water more slowly and waste less to runoff — worth the slightly higher price per head.

Key takeaways

Corners are the hardest part of a rectangle: a 90° head in every corner, aimed diagonally inward, is non-negotiable.

Keep head spacing at or below the radius (head-to-head) and keep each circuit under 80% of your available flow.

Measure pressure first — it sets the radius you can use — then let SprinklerMap turn the layout into an exact materials list before you buy anything.