How to design garden irrigation, step by step
12 July 2026 · 11 min read
A good design is 80% of a working irrigation system. This guide walks the whole process — from measuring your water supply to sizing pipe — following the same rules you will find in the Hunter and Rain Bird design catalogues.
Step 1. Measure your water source
Before you draw a single sprinkler you need two numbers: the pressure and the flow your supply can deliver. Together they decide how many sprinklers can run at once — and therefore how many zones the garden splits into.
- Static pressure is measured with an inexpensive gauge screwed onto the garden tap. Rotary sprinklers need at least 2.1 bar at the sprinkler to work properly — which, after losses in the pipe and solenoid valve, means roughly 3 bar at the supply point.
- Flow comes from the bucket test: open the tap fully and time how long a 10-litre bucket takes to fill. 20 seconds = 30 l/min = 1.8 m³/h, a typical domestic connection.
Step 2. Draw the plot to scale
You need a dimensioned plan: lawn boundaries, house, patio, paths, beds, trees and the location of the tap or well. Squared paper at 1:100 is enough — or an online editor where you draw the same thing with a mouse and get the calculations for free. What matters is that the dimensions are right: half a metre of error across a lawn can change the entire sprinkler layout.
Step 3. Place sprinklers head-to-head
The single most important rule in the whole design: each sprinkler's stream must reach the next sprinkler. That makes spacing equal to the throw radius — a sprinkler with a 5 m throw goes every 5 m. It sounds wasteful, but every sprinkler waters most weakly at the end of its range, and overlapping streams are what evens the precipitation out. There is more detail in the article on head-to-head spacing.
The placement order, as Hunter's own manuals teach it:
- Corners — 90° arc sprinklers, watering inwards across the lawn.
- Edges — 180° sprinklers along the sides, evenly spaced no further apart than one radius.
- Interior — only once the distance between rows exceeds the throw do you add full 360° heads in the middle.
Sprinklers always aim inwards. Watering the fence, the render and the pavement is the most common and most visible mistake in amateur installations.
Step 4. Choose sprinkler and nozzle types
| Type | Throw | Use case |
|---|---|---|
| Fixed spray nozzles | 1.5–5.5 m | small and narrow lawns, verges |
| Rotary nozzles (e.g. MP Rotator) | 2.5–10.7 m | medium lawns; low precipitation rate means fewer zones on a weak supply |
| Geared rotors | 5–15 m | large lawns, above roughly 200 m² |
| Drip line | — | beds, hedges, trees, narrow planting strips |
One iron rule: never mix nozzle types within a zone. A fixed spray applies around 40 mm/h and a rotary nozzle around 10 mm/h — share a zone between them and one area drowns before the other has had a drink.
Step 5. Split the system into zones
Add up the flow of every sprinkler (the manufacturer's tables give it per nozzle, arc and pressure). If the total exceeds your supply flow, split it into zones opened in sequence by solenoid valves. On a 1.8 m³/h connection with rotary nozzles, a typical 300–500 m² garden comes out at 3–5 lawn zones plus a separate drip zone.
- Plan zones so that everything inside one has the same precipitation rate.
- Beds and lawn always go on separate zones — different water needs and run times.
- Leave 10% flow headroom; the filter and meter take their own share.
Step 6. Design the pipework and valve box
PE (polyethylene) pipe with compression fittings is the usual choice. Diameter follows flow — the goal is to keep water slower than about 1.5–2 m/s, or pressure loss eats into sprinkler throw:
| PE diameter | Practical max flow | Typical role |
|---|---|---|
| Ø25 mm | up to ~1.4 m³/h | short branches to sprinklers |
| Ø32 mm | up to ~2.4 m³/h | zone laterals, smaller mains |
| Ø40 mm | up to ~4.2 m³/h | main line on a high-flow supply |
Group the solenoid valves into one box near the water source (up to four valves per box) so servicing happens in one place instead of across the whole garden. Run the control cable from the box to the controller, and bury pipe 25–30 cm deep, below the reach of a garden fork or aerator.
Step 7. Controller and schedule
The controller opens zones in sequence, so you simply need a model with at least as many stations as zones, plus a spare. A lawn at peak season needs roughly 25–35 mm of water per week; with rotary nozzles that usually works out at 2–3 waterings of 30–50 minutes per zone, always at dawn. A rain sensor — or a controller with an online weather feed — pays for itself in a single wet month.
The most common design mistakes
- Eyeballed spacing wider than the radius — dry patches in the first hot week.
- Mixing fixed spray and rotary nozzles in one zone.
- Too many sprinklers per zone — pressure drops and the corners stop reaching.
- No sprinklers in the corners ("the middle will cover it" — it will not).
- Ø25 pipe used as a 40-metre main — half the pressure stays in the pipe.
- Skipping drainage or a compressed-air blow-out before winter.
Is it worth designing yourself?
The design itself takes an hour or two on paper, as long as you hold to the rules above. You can also draw the plot in IrriForge: the editor places sprinklers head-to-head, splits the system into zones with a pressure check, and generates a shopping list with pipe lengths. Free to start, no account needed. Before you begin, it is worth confirming your supply can carry the design at all — that is what the pressure and flow guide is for.