Fixture unit method — size your branch drain, stack, or building drain to the actual load. Based on AS/NZS 3500.2.
The fixture unit method in AS/NZS 3500.2 assigns each plumbing fixture a Drainage Fixture Unit (DFU) value based on its discharge rate and frequency of use — a WC is 8 DFU, a shower is 3 DFU, a hand basin is 1 DFU. The total DFU load on a drain determines the minimum pipe size and minimum grade to maintain self-cleansing velocity.
The critical rule: a WC always requires a minimum 100mm branch drain regardless of how low the total DFU count is. You cannot undersize a pan outlet connection. The tool enforces this automatically and shows the watchouts for each drain type — grade too flat, oversizing, venting distances, and access point requirements.
Use this when sizing drainage for a new bathroom, extension, or fitout — or to check an existing drain configuration. The tool handles branch drains (horizontal), drainage stacks (vertical multi-storey), and building drains (under-slab runs to the sewer connection).
Enter how many of each fixture type drain to this pipe. Leave at zero if not connected.
Sanitary
WC / Toilet
8 DFU each
Urinal (wall-hung)
4 DFU each
Urinal (stall / trough)
4 DFU each
Bathroom
Shower
3 DFU each
Bath / bathtub
3 DFU each
Hand basin / vanity
1 DFU each
Bidet
1 DFU each
Kitchen
Kitchen sink (domestic)
2 DFU each
Kitchen sink (commercial)
4 DFU each
Dishwasher (domestic)
2 DFU each
Laundry
Laundry tub
3 DFU each
Washing machine
3 DFU each
Other
Floor waste / floor drain
2 DFU each
Slop sink / cleaner's sink
8 DFU each
Drinking fountain / bubbler
0.5 DFU each
Use the grade you can actually achieve on site, not the ideal.
Venting — Check These Before You Size the Drain
Every fixture trap needs venting or self-sealing
Each trap must be vented to prevent siphonage. Self-sealing traps (HepVO type) are an alternative where venting is impractical. Stack vented systems must be designed per AS/NZS 3500.2.
Max distance from trap to vent
Trap arm length (trap outlet to vent pipe) must not exceed the limits in AS/NZS 3500.2. Generally: 32mm trap = 1.5m, 40mm = 1.8m, 50mm = 3.0m, 65mm+ = 3.5m. Exceeding these causes trap siphonage.
Combined waste/vent pipe restrictions
Pipes that act as both drain and vent (dry-vented systems) have specific gradient and diameter rules. Don't assume a drain can also vent without checking AS/NZS 3500.2 Section 5.
WC branch must connect to stack at correct angle
WC branch connections to the stack must use an appropriate fitting — usually a 45° or 90° long radius WC pan connector. Short-radius bends below WC pan outlet are not permitted.