Flow and pressure calculation: example with real data

Step 1: measure the pressure

Required tool

Pressure gauge with a 1/2" threaded fitting for an outdoor tap. Cost: €10–18 at a hardware store or irrigation shop. Screw it onto the garden tap (not an indoor tap).

Static measurement

All taps in the house closed. Open the garden tap slowly, wait 10 seconds. Reading: 2.5 bar.

Dynamic measurement

With the gauge still attached, open a tap inside the house and re-read after 15 seconds. Reading: 2.1 bar.

The difference (0.4 bar) indicates the load on the water mains. The design value is always the dynamic pressure: 2.1 bar.

Step 2: measure the maximum flow rate

Bucket method

Remove the gauge, open the tap fully, fill a 10-liter bucket and time it.

Result: 10 liters in 33 seconds.

Calculation: (10 liters / 33 seconds) × 60 = 18.2 L/min

This is the maximum available flow. The system cannot use more than this — in practice, at most 75–80% for margin: 18.2 × 0.78 = 14.2 L/min as the operating limit.

Step 3: calculate head loss

Components and estimated losses

• PE 25 mm pipe, 15 m at 12 L/min: loss about 0.12 bar
• 1" mesh filter (clean): 0.07 bar
• Open 1" solenoid valve: 0.15 bar
• 3 T-fittings on the main pipe: 0.04 bar
• 6 branches of 16 mm PE, 0.5 m, to sprinklers: 0.02 bar

Estimated total loss: 0.40 bar

Pressure available at the farthest sprinklers: 2.1 − 0.40 = 1.7 bar

At 1.7 bar, a Hunter PGP-ADJ sprinkler delivers a radius of about 3.8–4.0 m (the nominal radius is rated at 2.5 bar = 4.5 m; at 1.7 bar it shrinks). This is the value to use in the design, not the nominal radius from the datasheet.

Step 4: calculate the maximum number of sprinklers per circuit

Data for the chosen sprinklers

Hunter PGP-ADJ at 1.7 bar: flow about 1.7 L/min (from the manufacturer's flow table at the actual operating pressure).

Calculation

Operating flow limit: 14.2 L/min
Flow per sprinkler: 1.7 L/min
Maximum number of sprinklers: 14.2 / 1.7 = 8.3 sprinklers

Rounded down to 8 sprinklers per circuit. With 6 sprinklers on the 10 × 8 m lawn, the circuit is well within limits (6 × 1.7 = 10.2 L/min < 14.2 L/min).

Check with multiple zones

If the lawn is large and 10 sprinklers are needed, split them into 2 circuits of 5. The two circuits run in sequence, so the maximum simultaneous flow remains that of a single zone: 8.5 L/min < 14.2 L/min.

Method summary

1. Design pressure = measured dynamic pressure
2. Estimated total losses = 0.3–0.6 bar (pipe + valve + filter)
3. Pressure at sprinklers = dynamic − total losses
4. Sprinklers per circuit = (maximum flow × 0.78) / sprinkler flow at actual pressure

Generate the project for your garden

These examples show typical scenarios. Use SprinklerMap to calculate a precise layout, pressures and materials list for your real situation.

Open SprinklerMap for free →