Why loading units and not "add up the taps"
Nobody runs every fixture in a house at once. If you sized a water service for the sum of every outlet flowing together you would put DN50 into a two-bathroom house and the water would sit in the pipe going stale.
The Australian method in AS/NZS 3500.1 handles that with loading units. Every fixture carries a number that reflects both how much it draws and how often it is likely to be running. You add them up, and a published curve converts the total into a probable simultaneous flow — the flow the service will realistically see at a busy moment, not the theoretical maximum.
The curve is not linear, and that is the whole point. Doubling the fixtures does not double the flow, because the odds of them all going at once fall as you add more.
Loading unit values
| Fixture | Loading units |
|---|---|
| Basin | 1 |
| Shower | 2 |
| WC cistern | 2 |
| Dishwasher | 2 |
| Kitchen sink | 3 |
| Laundry tub | 3 |
| Washing machine | 3 |
| Hose tap | 5 |
| Bath | 8 |
A bath is eight and a basin is one, which surprises people until you watch a bath fill. Hose taps at five are the ones most often left off a takeoff and they are not small.
A worked example
A modest house — one bathroom, kitchen, laundry:
- Basin 1 + kitchen sink 3 + shower 2 + WC cistern 2 + laundry tub 3 = 11 loading units
- Probable simultaneous flow: 0.25 × 110.6 = 1.05 L/s
- At a 2 m/s design velocity that needs about 527 mm² of bore
- DN25 has 483 mm² — just short. DN32 has 794 mm², so the tool suggests DN32, running at 1.33 m/s
Now look at what the velocity setting does. Put that same 1.05 L/s down DN25 and it runs at 2.18 m/s — only just over the conservative 2 m/s default. Plenty of short domestic services do run there. Raise the design velocity in the calculator and it will pick the smaller pipe. That is the honest picture: the answer is a function of the velocity you are willing to accept, not a single fixed truth, and higher velocity buys you a cheaper pipe at the cost of noise, water hammer and long-term erosion of the bore.
Flow a pipe carries at 2 m/s
| Size | Internal bore | Flow at 2 m/s | Loading units it covers |
|---|---|---|---|
| DN15 | 13.8 mm | 0.30 L/s | about 1 |
| DN20 | 19.0 mm | 0.57 L/s | about 4 |
| DN25 | 24.8 mm | 0.97 L/s | about 10 |
| DN32 | 31.8 mm | 1.59 L/s | about 22 |
| DN40 | 38.7 mm | 2.35 L/s | about 42 |
| DN50 | 48.5 mm | 3.69 L/s | about 89 |
Bores are nominal figures for common Australian copper and PEX sizes — check the actual bore of the product you are running, because it varies by material and pressure class, and PEX in particular has a noticeably smaller bore than copper of the same nominal size.
What this calculator does not do
It sizes on flow. The real method has a second half, and it is the half that fails jobs: pressure.
You start with the pressure available at the meter, subtract the head loss through every metre of pipe, every elbow, every valve, the meter itself, any filter, and the static lift up to the highest outlet. What is left at the most hydraulically disadvantaged fixture — usually the upstairs shower furthest from the meter — has to be at least the minimum residual pressure the standard requires, commonly taken as 150 kPa.
A pipe can be perfectly adequate for the flow and still leave that shower at 90 kPa. The fix is usually a size up on the run, fewer fittings, or a pump — and you cannot tell which without the site's measured supply pressure, which this tool does not ask for and could not guess.
Practical notes
- Measure the supply pressure before you quote. A gauge on a hose tap takes two minutes and can change the whole design. Note the time of day, because street pressure moves.
- Count the hose taps. Five LU each, and customers add them later.
- Size for what is coming. If there is an obvious future ensuite or a granny flat, the extra cost of the next size up on the main run is small compared to redoing it.
- Fittings are hidden length. Every elbow is worth a couple of metres of straight pipe in head loss. A run with fourteen bends is not the same as a run with four.
- Noise. Anything much over 2 m/s starts to be audible in a wall cavity, and customers who can hear the plumbing tell people about it.
Quoting a water service
Pipe is cheap. The trench, the wall chases, the fittings count and the reinstatement are not. Get the size right first, then price the labour that actually costs money. Water service work is licensed plumbing work in every state and territory — this is an estimating aid for the trade, not a design certificate.
Sized the pipe. Now price the run.
Yamate turns metres of pipe, fittings and hours into a quote the customer accepts on their phone, then an invoice, then a payment — with GST handled and the follow-ups chased for you.
Try Yamate free → Free to try · no card · sign in with a link, no password to rememberWater pipe sizing FAQ
What size water pipe do I need for a house?
A one-bathroom house with kitchen and laundry is about 11 loading units, or roughly 1.05 L/s of probable simultaneous flow. At a conservative 2 m/s that points to DN32, though plenty of short domestic services run DN25 at about 2.2 m/s. Enter your own fixtures above.
What is a loading unit?
A number given to each fixture that reflects how much water it draws and how likely it is to be running at the same time as everything else. You add them up and a published curve converts the total into a probable simultaneous flow, which is far less than the sum of every outlet.
How many loading units is a bath?
Eight, the largest of the common domestic fixtures. A shower is 2, a basin is 1, a kitchen sink or laundry tub is 3, a WC cistern is 2, and a hose tap is 5.
Does this calculator check water pressure?
No, and that matters. It sizes on flow only. The full AS/NZS 3500.1 method also works out head loss from the meter to the most disadvantaged fixture and requires a minimum residual pressure there, commonly 150 kPa. You need the site's measured supply pressure for that, so do it separately.
What velocity should water pipe be sized at?
2 m/s is a conservative working default that keeps noise, water hammer and long-term erosion down. Higher velocities let you use a smaller pipe and are common on short domestic runs, but the pipe gets louder and wears faster.
Is PEX the same bore as copper?
No. PEX of the same nominal size has a noticeably smaller internal bore than copper, so it carries less at the same velocity. Use the actual bore from the manufacturer's data rather than the nominal size when the run is tight.
Can I size and install water pipe myself?
No. Water service and internal plumbing work in Australia is licensed work. This calculator is a sizing aid for licensed plumbers, not a DIY guide.
Other calculators you'll want
General information for Australian trades, and an estimate only — not tax, financial or legal advice. It is not engineering, building or design advice either: a calculator works out a quantity from the figures you enter, and anything that needs sign-off still needs your engineer, certifier or building surveyor. Loading-unit values and the loading-unit-to-flow relationship follow published Australian guidance on AS/NZS 3500.1 and can change between editions; the flow curve used here is a close approximation over the domestic range, not a reproduction of the standard's table. Pressure, head loss and residual pressure at the most disadvantaged fixture are NOT calculated here. Internal bores are nominal and vary by material and pressure class. Plumbing is licensed work — this is an estimating aid, not a hydraulic design.