Sanitary Drainage Pipe Sizing Calculator

Under AS/NZS 3500.2 Table 3.2, a DN100 pipe can carry up to 80 DFU on a horizontal branch drain and up to 150 DFU on a building main drain. A typical Australian 3-bedroom home generates 20–30 DFU, meaning a single DN100 main drain easily accommodates standard residential loads.

DFU stands for Drainage Fixture Unit (also referred to as Loading Units under AS/NZS 3500.2). It is a numeric value assigned to each plumbing fixture representing its probable peak wastewater discharge load on the drainage system. The total DFU count determines the minimum pipe diameter required.

A typical 3-bedroom Australian home with 2 bathrooms, a kitchen, and a laundry totals around 20–30 DFU. This sits comfortably within the DN100 branch drain limit of 80 DFU and building drain limit of 150 DFU.

DN150 is required when fixture unit loading exceeds DN100 capacities (more than 80 DFU on a branch drain or more than 150 DFU on a main drain). DN150 supports up to 350 DFU on horizontal branches and 700 DFU on building main drains, making it standard for commercial developments, apartment complexes, and schools.

Yes. A standard residential bathroom with a dual-flush toilet (3 DFU), basin (1 DFU), and shower (2 DFU) totals 6 DFU. Since a DN100 branch drain allows up to 80 DFU under AS/NZS 3500.2, it can easily serve 4 or more full bathrooms on the same run.

Self-cleansing velocity is the minimum flow speed needed to keep solid waste moving without settling out. AS/NZS 3500.2 requires a minimum velocity of 0.6 m/s at half-pipe depth for sanitary drains.

No. AS/NZS 3500.2 requires a minimum DN100 pipe for any drain line connected to a toilet (WC). DN65 has a maximum capacity of 3 DFU, and a single toilet is 3–4 DFU on its own.

A fixture discharge pipe conveys waste from an individual fixture (e.g., 40mm pipe from a basin or 50mm from a shower) to a branch drain or stack. A branch drain is a horizontal pipe that collects discharge from multiple fixture discharge pipes and carries it to the main building drain or stack.

Commercial kitchens typically require minimum DN100 branch drains connected through trade waste grease arrestors. Under AS/NZS 3500.2, commercial fixtures have higher DFU ratings (commercial sinks 3–4 DFU, dishwashers 3 DFU, floor wastes 2–3 DFU). If total kitchen DFU exceeds 80, a DN150 line must be installed.

A branch drain is a horizontal pipe collecting waste from individual fixtures or groups of fixtures that feeds into a stack or main line. The building drain is the horizontal main pipe running from the last stack to the boundary sewer connection.

Yes, mostly in multi-storey residential and commercial buildings where acoustic attenuation and passive fire protection are required.

SN refers to Stiffness Number (kN/m²). SN4 is standard for residential drainage under normal soil backfill. SN8 is heavy-duty pipe required under driveways, car parks, and vehicle traffic areas.

On a branch drain, DN100 handles up to 80 DFU. Since a dual-flush toilet is 3 DFU, a single DN100 line can theoretically handle up to 26 dual-flush toilets.

Yes. All sanitary drainage work must be performed by a licensed plumber/drainer and submitted for council or water authority compliance inspection.

Use n = 0.009 for uPVC and PVC-U DWV pipes. For cast iron use n = 0.012, and for vitrified clay (VC) use n = 0.013.