Standard Pipe Size for Toilet Drains: DN100 Explained

If you've ever wondered why your plumber insists on a specific pipe size for the toilet — and won't budge on it — here's the short answer: the standard pipe size for toilet drains is DN100 everywhere in Australia, and that's not negotiable. DN100 means a nominal diameter of 100mm, which gives waste enough room to clear the pan cleanly on every flush. In my experience, cutting corners on pipe sizing is one of the fastest ways to end up with a blocked drain and a failed inspection. The rules are set out in AS/NZS 3500.2:2025 — Sanitary Plumbing and Drainage, and every licensed plumber works to them.

An educational guide explaining correct drain line sizes for toilets, showers and sinks, helping Australian readers understand why a 100mm pipe diameter is the standard requirement for toilet drainage
Video Credit: The Excellent Laborer

What Is DN100 and Why It Is the Standard Pipe Size for Toilet Drains in Australia

DN stands for Diameter Nominal and is the ISO pipe size designation used throughout Australian plumbing. A DN100 pipe size means that the pipe has a nominal diameter of 100mm, which is the standard pipe size for toilet drains in Australia.

Just to make this clear — a nominal pipe diameter is not the actual internal or external diameter. A DN100 uPVC pipe will have an outside diameter of 110mm and a bore slightly less than that depending on the wall thickness of the pipe.

Why do you need a 100mm pipe for a toilet drain? You'll often see 80mm or even 50mm pipework used for other drains, such as for a basin or a shower. But there's something else going down a toilet drain pipe, right? And what goes down a toilet drain needs to pass through the pipe with enough room for the pipe to not block up. A 4-star WELS toilet uses 4.5 litres on a full flush and 3 litres on a half flush. That volume of water needs to leave the pan fast enough to take the waste with it, not leave any behind. And that requires a larger internal bore to pass through. A smaller bore pipe size will give you a clog in no time flat. I've seen it, and I don't want to talk about it.

Typically, toilet drain pipes in Australia will be uPVC or ABS, both of which are light, corrosion resistant, and easy for a licensed plumber to fit. No matter the material, the standard pipe size for toilet drains is DN100 everywhere in Australia.

Diagram of toilet with labelled DN100 drain pipe showing Australian compliance standards

Australian Standards and Regulations Governing Toilet Drain Pipe Sizing

There are many standards to follow when it comes to toilet drains but the main standard for toilet drain pipe sizing is AS/NZS 3500.2:2025 — Sanitary Plumbing and Drainage. This document lays out the rules for connecting a toilet pan, the required fall (gradient) for the drain, vent pipe sizing, and how to join different lengths of drain pipe together. This is the standard a licensed plumber will use to install your toilet, and the one that the council building inspector will check against when they come to inspect your plumbing.

The National Construction Code requires that all sanitary drainage works are in compliance with AS/NZS 3500.2, which is mandated through state and territory building regulations. Choosing the wrong toilet drain pipe size may void your home insurance policy, and can lead to a failed council building inspection — the last thing you want after you finish tiling your new bathroom.

All plumbing products used in a toilet installation need to be WaterMark certified. WaterMark is the mandatory national certification scheme for plumbing and drainage products, administered by the Australian Building Codes Board. Any fitting or product installed by a licensed plumber should carry a WaterMark symbol, regardless of how cheap it looks.

Isometric cutaway of house bathroom plumbing draining into underground septic tanks

How DN100 Pipe Works in a Typical Toilet Drainage System

So here's what happens when you flush the toilet. Waste leaves the toilet pan and passes through the trap — a p trap pan connector or s trap depending on the toilet layout — then into the drain line joining the trap to the 100mm drain pipe. The 100mm drain pipe travels under the floor or inside the wall cavity to the sanitary stack.

Gradient is important. According to AS/NZS 3500.2, a 100mm drain pipe needs to be sloped between 1:40 and 1:60. If the slope is not steep enough, waste will collect in the pipe. If the slope is too steep, the water will move faster than the waste, leaving the waste behind. Either way, you're looking at a slow drain, odour, and blockage. Negative pressure will pull water out of the trap seal, creating a pathway for sewer gas to enter the house. Every toilet on the Australian market that is 4-star WELS compliant to AS/NZS 6400:2016 comes with a DN100 drain connection, and the flush volume is calibrated for that pipe diameter at the gradients given in the standard.

Diagram comparing S-trap floor outlet and P-trap wall outlet toilet drain configurations with measurements

Choosing the Right Trap and Connector for Your DN100 Toilet Drain

It depends on whether the drain outlet exits the floor or the wall. If the drain outlet exits the floor, an s trap is used. The typical setout is 100–120mm from the finished wall to the centre of the pipe. If the drain outlet exits the wall, a p trap pan connector is used. The typical setout is 185mm from the finished floor to the centre of the pipe.

Check the setout measurement before purchasing a pan connector. Some pan connectors are adjustable and can cater for different offset distances, which is helpful if the rough-in is a few millimetres out — common in older houses. All traps and pan connectors must be WaterMark certified before being installed in an Australian drainage system.

Plumber tightening chrome sink trap with orange adjustable spanner under basin

Installation Tips, Common Mistakes, and Water Supply Connections

Make sure your rough-in measurements are spot on before purchasing any products. This is the most common problem I see — buying a pan connector that is too short, or a suite that sticks out from the wall because the setout wasn't measured properly.

On the water supply side, the toilet cistern tap (or isolation valve) needs to be the correct size and WaterMark certified. The toilet cistern tap and supply line are entirely separate from the DN100 drain system, but are just as important to get right. A poorly connected toilet cistern tap, or one that isn't WaterMark approved, could result in a slow leak behind the wall.

All joints in the DN100 system should either be solvent welded or sealed with a rubber ring to AS/NZS 3500.2. Any leaks must be fixed before floors or walls are closed up.

Once the work is complete, your plumber should provide a certificate of compliance. Connecting a toilet to a waste pipe or water supply is licensed plumbing work in all Australian states — do not attempt to cut, join, or alter DN100 drain pipes yourself. Check your state/territory requirements, as licensing rules vary.

The most common mistakes are:

• Choosing the wrong trap depth

• Using the wrong offset connector for the setout distance

• Using pipe or fittings that aren't WaterMark approved

All of these are costly to fix after your bathroom has been tiled. If you're doing a reno, have your rough-in measurements checked by your licensed plumber before the tiles go in. The standard pipe size for toilet installations is DN100 — getting that right from the start saves significant rework.

References

AS/NZS 3500.2:2025 Plumbing and Drainage — Sanitary Plumbing and Drainage, Standards Australia

AS/NZS 6400:2016 Water efficient products — Rating and labelling (incorporating Amendment No. 1:2022 and Amendment No. 2:2022), Standards Australia

WaterMark Certification Scheme, Australian Building Codes Board

State and territory plumbing licensing authorities (Building and Plumbing Commission Victoria; Building Commission NSW / NSW Fair Trading; Queensland Building and Construction Commission; Plumbers Licensing Board Western Australia (administered by Building and Energy); Consumer, Building and Occupational Services Tasmania; Access Canberra ACT; Office of the Technical Regulator / Consumer and Business Services South Australia; Plumbers and Drainers Licensing Board Northern Territory)

FAQs

Can a DN100 toilet drain share the same pipe run as a basin or shower drain?

Yes, but only under specific conditions set out in AS/NZS 3500.2 — the combined drain must still maintain the correct gradient and adequate capacity for the total fixture unit load. Your licensed plumber will calculate this before tying fixtures into a shared line, and the toilet connection must always enter the stack or shared drain at the correct angle to avoid backflow risk into the smaller-bore branches.

How long does a typical toilet rough-in and drain installation take on a bathroom renovation?

For a straightforward like-for-like replacement, a licensed plumber can usually complete the drain rough-in and pan connector installation in two to four hours. A full repositioning of the toilet — which requires rerouting the DN100 drain line — can add a full day or more depending on floor construction and access.

Does the DN100 drain pipe size change if I install a wall-hung toilet rather than a floor-mounted suite?

No — the drain remains DN100 regardless of the toilet style. Wall-hung suites use an in-wall cistern frame with a concealed pan connector, but the waste outlet still connects to a DN100 line behind the wall frame.