Mains water for garden irrigation: pressure and flow
19 August 2026 · 9 min read
An entire irrigation design rests on two numbers you can measure yourself in a quarter of an hour: the pressure and the flow of your supply. They determine how many sprinklers can run at once, how many zones the garden needs and what pipe you buy. Skip this step and everything after it is guesswork.
Why these are two different numbers
People conflate them, and that is the root of most failed installations.
- Pressure is the force the water pushes with. It decides whether a sprinkler pops up at all and how far it throws. Measured in bar.
- Flow is how much water actually moves per unit of time. It decides how many sprinklers you can run simultaneously. Measured in litres per minute.
You can have high pressure and poor flow — typically with a thin supply pipe or a small water meter. One sprinkler then works perfectly while five together barely dribble. That is why you measure both.
Measurement 1. Static pressure
You need a pressure gauge that threads onto the garden tap. It is inexpensive and the best-spent money in the whole project.
- Screw the gauge onto the garden tap.
- Close every other outlet in the house — washing machine, dishwasher, shower. You are measuring static pressure, meaning at zero flow.
- Open the tap fully and read the gauge.
- Repeat in the evening, between 6 and 9 pm. That is peak demand on the network and when pressure is lowest — and irrigation usually runs early morning or late evening anyway.
What counts as a good result
| Reading at the tap | What it means | What to do |
|---|---|---|
| below 2 bar | Not enough for pop-up sprinklers — they will not deploy properly | Tank with a pump, or a drip-based system |
| 2–3 bar | Marginal. It works, but only with short runs and small zones | More, smaller zones and larger pipe diameters |
| 3–5 bar | The comfortable range — the usual design target | Design normally |
| above 5 bar | Too much: misting, faster seal wear, water hammer | Fit a pressure regulator |
Measurement 2. Flow — the bucket test
The simplest and surprisingly accurate measurement in irrigation design. You need a bucket of known volume and the stopwatch on your phone.
- Take a 10-litre bucket (or any bucket of known volume).
- Open the garden tap fully, with all other outlets closed.
- Time how long the bucket takes to fill to the brim.
- Convert: flow [l/min] = 600 ÷ time in seconds. A 10-litre bucket filled in 30 s gives 20 l/min.
- Repeat three times, including in the evening, and design around the worst result.
| Time to fill 10 l | Flow | Sprinklers per zone* |
|---|---|---|
| 20 s | 30 l/min | about 12 |
| 30 s | 20 l/min | about 8 |
| 45 s | 13 l/min | about 5 |
| 60 s | 10 l/min | about 4 |
| 90 s | 6.7 l/min | about 2–3 — consider drip instead |
*Estimated with nozzles using roughly 2 l/min and 80% of the available flow. The real figure depends on the specific nozzles and arcs — spacing is covered in full in sprinkler spacing: head-to-head coverage.
What limits flow on a mains supply
If the bucket test disappointed you, the bottleneck is usually one of four things — and it is not always the mains:
| Component | Typical problem | Can it be improved? |
|---|---|---|
| Service connection | Diameter too small to serve house and garden at once | Replacing it is an investment with utility paperwork attached |
| Water meter | The most common real bottleneck — it has its own flow limit | A larger one can sometimes be agreed; requires an application |
| House filter and regulator | A clogged cartridge can cut flow by tens of percent | Yes, and for free — replace the cartridge and measure again |
| Run to the garden tap | A long, thin pipe with many elbows | Yes — a new, larger dedicated branch for irrigation |
Before concluding that your water is weak, change the filter cartridge and repeat the bucket test. It is the most common and cheapest cause.
From measurement to zones — how it works out
An example for a typical plot. Measured: 3.2 bar and 20 l/min, with 400 m² of lawn.
- Flow budget per zone: 80% of 20 l/min = 16 l/min.
- Demand: with nozzles at roughly 2 l/min each, 400 m² needs somewhere in the low twenties of sprinklers — say 24, so about 48 l/min in total.
- Zone count: 48 ÷ 16 = 3 zones, run in sequence.
- Pipe diameters: at 16 l/min per zone, Ø20 pipe (good to 30 l/min) covers the branches, and the main line goes one size up.
| PE pipe | Maximum flow |
|---|---|
| Ø20 mm | 30 l/min |
| Ø25 mm | 50 l/min |
| Ø32 mm | 83 l/min |
These limits come from the permissible flow velocity — exceed them and you get excessive pressure loss and water hammer. Branches are sized from the far end back: the pipe feeding the last sprinkler is always Ø20, and diameter grows towards the source as the accumulated flow grows.
This is exactly the arithmetic IrriForge does for you in the editor: you enter the pressure and flow measured above, and it selects nozzles, splits the garden into zones that fit your flow, and checks that the required pressure still remains at the furthest sprinkler.
A separate garden meter — worth doing the maths
Water used for irrigation never enters the sewer, yet without a sub-meter many tariffs charge you for treating it exactly as if it had. An additional garden meter can let you deduct that volume from the wastewater charge.
With automatic irrigation using tens of cubic metres across a season, the difference can be significant. The rules, required seals and paperwork are set by your local water utility and differ substantially between areas — call and ask about the terms before ordering an installation. It is one phone call, and it decides whether the meter pays for itself.
When the mains is not enough
Options, cheapest first:
- More, smaller zones. The cost is extra solenoid valves and controller stations. Total watering time gets longer, but the system works correctly.
- Convert some sprinkler areas to drip. Beds, hedges and shrubs do not need sprinklers — drip line uses a fraction of the water and runs at low pressure.
- A larger dedicated branch off the house plumbing for irrigation, if the bottleneck is a long thin run to the garden tap.
- A buffer tank with a pump. The tank fills slowly over a day and the pump supplies flow and pressure the mains cannot. The most expensive option, but it removes the dependency on the network and lets you use rainwater.
Frequently asked questions
What water pressure does garden irrigation need?
A sprinkler needs a minimum of about 2.1 bar measured at the sprinkler itself, and MP Rotator nozzles are at their best at 2.8 bar. Because pipe, solenoid valve and filter each take a share, you want roughly 3 bar at the supply point. Above 5 bar at the supply, fit a pressure regulator.
How do I measure flow without a professional?
The bucket test: open the garden tap fully, time how many seconds a 10-litre bucket takes to fill, and divide 600 by that time. The result is flow in litres per minute. Repeat three times and use the worst result.
How many sprinklers can run on one zone?
As many as fit inside about 80% of your measured flow. At 20 l/min with nozzles using roughly 2 l/min each, that is eight sprinklers per zone. The rest of the garden goes into further zones, which run in sequence rather than together.
Is a separate garden water meter worth it?
Often yes if you irrigate automatically all season, because water used for irrigation never enters the sewer, and a sub-meter can let you avoid paying wastewater charges on it. The rules and paperwork are set by your local water utility and vary widely, so confirm the terms locally before ordering an installation.
What if mains pressure is too low?
First split the system into more, smaller zones — that costs nothing beyond extra solenoid valves. If that is not enough, consider a buffer tank with a pump, or converting some sprinkler areas to drip line, which runs at much lower pressure.
Summary
Measure pressure with a gauge and flow with a bucket, repeat both in the evening, take the worst result and leave 20% headroom. Those two numbers determine the number of zones, the nozzle selection and the pipe diameters — that is, the whole design. Everything else, sprinkler placement included, follows from them. The rest of the process is covered in how to design garden irrigation step by step.