Pipe Sizing – Fact or Fiction?


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Pipe Sizing – Fact or Fiction?

What size pipe should I use?

What do the stripes mean?
Does a smaller pipe give me more pressure?
What is pipe friction? 

So many questions! And so many myths, rules of thumb, misinformation and confusion. Let’s go through it step by step and lay out the facts.

Pipe types.
Let's keep it simple and just talk about polyethylene (poly, or PE) pipe. But the principles are the same for PVC pipe, steel pipe, garden hose and all types of pipe.

The main types of poly pipe are:
Low Density, Rural and Metric. Common sizes for Low Density pipe are 13, 19 and 25mm. It is generally rated at 40-50psi. Low density pipe does not normally have a stripe.

Rural poly pipe is an old standard which remains popular with farmers and irrigators. It is often referred to as B Class poly and generally has a green stripe. Rural pipe sizes are 3/4”, 1”, 1 1/4”, 1 1/2” and 2”. It is rated to 800kpa (115psi).

There are several classes within the Metric pipe range each with a different pressure rating and wall thickness. It is often required for domestic plumbing with a mains supply. Metric pipe ranges from 16mm up to 110mm and beyond (16, 25, 32, 40, 50, 63, 75, 90 and 110mm) and have a blue stripe. They typically have pressure ratings of 800kpa up to 1600kPa (PN8, PN10, PN12.5, PN16).

Pipe Sizes.
This can get confusing. The nominal size has little resemblance to the actual measured size. Also, Low Density and Rural poly pipe are measured by their inside diameter, while Metric poly is measured by the outside diameter.  For example, 1” Rural poly pipe has an OD of 29mm and an ID of 24mm, while 25mm PN12.5 Metric pipe has an OD of 25mm and an ID of 20mm. If you aren’t sure cut a small piece and bring it in.

Here is where it all comes together. Friction is the resistance that a pipe imparts on water as it moves through. It’s a bit like putting your hand out the window of your car. When you are stopped there is no resistance, but the faster you go, the more resistance there is. It is also similar to voltage drop in an electrical cable.

In essence the faster water moves through the pipe, the more resistance there is. The resistance is called pressure loss or pipe friction. For example, if we are moving 100L per minute through both a 2” pipe and a 3/4” pipe, the water will be moving faster in the 3/4” pipe – and hence there will be more pressure loss (friction) in the 3/4” pipe.

Size matters.
Additionally, it makes sense that water flows quicker through a 1m hose than a 1000m hose. Friction works on a per meter basis. So, the smaller the pipe  and the longer the pipe, the more friction (and thus less flow) will result.

At McLaren Vale Irrigation, we have charts and software to help work through this, so feel free to come in and get the right pipe first time. We also stock and supply products, fittings and accessories from Australia's leading brands:

  • HR Products
  • Iplex Pipelines
  • Philmac Australia
  • Toro Australia
  • Vinidex


McLaren Vale Irrigation