Water Pipe Sizing Calculator

Why Velocity Matters Just As Much As Pressure

When most blokes think about sizing a water service, they just think about delivering enough pressure. But AS/NZS 3500.1 puts a hard cap on water velocity—generally 3.0 m/s for most standard domestic pipes, though many aim for under 2.4 m/s just to be safe.

Pushing water too fast causes water hammer, erodes copper pipe fittings from the inside out (impingement attack), and creates a racket when the tap is turned on. If you're teeing off a high-pressure mains supply, smaller pipes might easily pass the pressure requirements, but they'll fail on velocity. Always check the velocity output when stepping down pipe sizes.

Friction Loss and Pipe Materials

Not all pipes are created equal. A DN20 Copper Type B pipe has a different internal diameter to a DN20 PEX pipe, meaning they carry different volumes of water and create different friction losses. We calculate friction loss using the Hazen-Williams formula.

Plastic pipes like PEX and Poly are incredibly smooth and have a high C-factor (around 150), meaning water slides through with minimal drag. Older galvanised pipes have a terrible C-factor (100) because they scale up and get rough on the inside. That’s why you lose so much pressure on an old galv service compared to a new copper or PEX run.

Worked Example 1: Sizing a PEX Run to a Granny Flat

Let’s say you’re running a line to a detached granny flat at the back of a block. You need a flow rate of 0.45 L/s to handle a shower, toilet, and sink running at the same time. The run is 45 metres long, and the mains pressure is a bit low at 300 kPa.

Maximum Flow Rates Before Hitting 3.0 m/s Limit

Here is a quick cheat sheet for the absolute maximum flow rate you can push through standard pipes before you hit the 3.0 m/s velocity limit set by the plumbing code.

Dealing With Static Head Loss (Elevation)

If you're running pipework up into a ceiling space or a multi-storey building, you have to account for gravity. For every metre you go up, you lose 9.81 kPa of pressure. We call this static head loss.

If you've got 500 kPa at the meter and you're roughing in a shower on the second floor (say, 6 metres high), you instantly lose about 59 kPa just fighting gravity before friction even enters the equation. Always punch the elevation change into the calculator if you are working on multi-storey jobs.