Question 1

What is a loading unit in plumbing?

Accepted Answer

A loading unit is a number assigned to each plumbing fixture based on how much water it draws and how often it runs. Under AS/NZS 3500.1, a basin gets 1 loading unit while a toilet gets 4 because the cistern refill draws a lot more water. You add up all the loading units in a building to work out how much flow the water main actually needs to deliver.

Question 2

How do you convert fixture units to litres per second?

Accepted Answer

AS/NZS 3500.1 has a conversion curve that takes your total loading units and gives you a 'probable simultaneous demand' flow rate in L/s. The idea is that not every fixture runs at the same time, so the curve accounts for probability. For example, 30 loading units converts to roughly 0.75 L/s, not the 3+ L/s you'd get if every tap was flat out.

Question 3

What is the difference between loading units and drainage fixture units?

Accepted Answer

Loading units (LU) measure how much water a fixture draws from the supply side — they're covered in AS/NZS 3500.1. Drainage fixture units (DFU) measure how much waste a fixture dumps into the drain — covered in AS/NZS 3500.2. Same fixture, but the LU and DFU numbers can be different because supply flow and drain flow aren't always equal.

Question 4

How many loading units does a toilet have in Australia?

Accepted Answer

A standard dual-flush toilet (6/3 litre) has 4 loading units under AS/NZS 3500.1. It also counts as 4 drainage fixture units on the waste side. Old single-flush toilets (11 litre) used to have higher values, but dual-flush is the norm across Australia now.

Question 5

What pipe size do I need for 20 loading units?

Accepted Answer

Twenty loading units converts to a probable simultaneous demand of roughly 0.55 L/s. A DN20 Copper Type B pipe can handle that comfortably with a velocity of around 1.33 m/s. If the pipe run is over 20 metres though, step up to DN25 to keep friction loss manageable.

Question 6

Do I need a hydraulic engineer to do fixture unit calculations?

Accepted Answer

For standard residential work, most plumbers size pipes themselves using AS/NZS 3500.1 tables and their own experience. But on commercial jobs — restaurants, offices, apartment blocks — councils and certifiers usually require a registered hydraulic consultant to produce a formal hydraulic services design report with fixture unit calculations, pipe sizing, and hot water system specs.

Question 7

How many fixture units can a 25mm pipe handle?

Accepted Answer

A DN25 Copper Type B pipe can handle a flow of about 1.24 L/s before hitting the 3.0 m/s velocity limit. That translates to roughly 55–60 loading units, which covers most standard houses. Keep in mind that friction loss over long runs will eat into your available pressure, so the practical limit depends on pipe length and mains pressure.

Question 8

What is probable simultaneous demand in plumbing?

Accepted Answer

Probable simultaneous demand is the realistic peak flow rate a building will actually draw at any given moment. It's based on the idea that in a house with 30 loading units, not all 30 units will be running at the exact same second. AS/NZS 3500.1 uses a statistical curve to predict the most likely peak demand — and that's the number you use for pipe sizing.

Question 9

Are fixture unit values different for commercial buildings?

Accepted Answer

Yes, commercial fixture unit values can differ from residential because the usage pattern is different. A restaurant kitchen sink runs harder and more frequently than a domestic one. Commercial buildings also include fixtures you don't find in houses — urinals, cleaner's sinks, grease traps. Your hydraulic engineer will use the specific values from AS/NZS 3500 for each commercial fixture type.

Question 10

How do I calculate drainage fixture units for a house?

Accepted Answer

Same process as loading units, but you look at AS/NZS 3500.2 instead of 3500.1. Each fixture gets a DFU number — toilets are 4, basins are 1, showers are 2. Add them up, then check the total DFU against the capacity of your main drain pipe. A DN100 sewer drain at 1:60 grade handles up to 90 DFUs, so most houses are well within limits.

Question 11

Does a hose tap count as a fixture unit?

Accepted Answer

Yes, a hose tap or garden tap counts as 3 loading units on the supply side because it can deliver a high flow rate when fully open. On the drainage side though, hose taps don't connect to the sewer, so they get no DFU value. If you've got multiple hose taps around the house, they add up quickly and can affect your main pipe sizing.

Question 12

What happens if I undersize the water main?

Accepted Answer

Undersizing means the pipe can't deliver enough flow when multiple fixtures run at once. You'll get complaints about low pressure in the shower when someone flushes the toilet. Velocity goes up, water hammer starts, and in copper pipes you can get erosion inside the fittings. It's a callback waiting to happen. Always calculate the loading units properly before committing to a pipe size.

Fixture Type	Water Loading Units (LU)	Drainage Fixture Units (DFU)	Typical Usage Notes
Wash Basin	1 LU	1 DFU	Standard 15mm cold/hot supply branch
WC / Toilet (Dual Flush 6/3L)	4 LU	4 DFU	Fast cistern refill draw
Shower	2 LU	2 DFU	Standard WELS 3-star (9 L/min rating)
Bath Tub	4 LU	3 DFU	High initial fill rate
Kitchen Sink	3 LU	3 DFU	Dual bowl kitchen setup
Dishwasher	2 LU	2 DFU	Intermittent solenoid valve draw
Washing Machine	3 LU	3 DFU	Cold & hot inlet valves
Laundry Trough	3 LU	2 DFU	Direct bypass wall outlet
Hose Tap / Garden Tap	3 LU	—	Full open garden hose valve

Plumbing Fixture Unit Calculator

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AS/NZS 3500 Loading Unit Values — Quick Reference

Worked Example: 4-Bedroom House With 2 Bathrooms

1 Tally the Fixtures

2 Sum Total Loading Units

3 Convert to Peak Flow Rate (L/s)

4 Determine Minimum Supply Pipe Size

Residential vs Commercial Fixture Unit Calculations

From Fixture Units to Pipe Sizing — Connecting the Dots

Frequently Asked Questions