What Are Drainage Fixture Units (DFUs)?
Every plumbing fixture connected to a sanitary drainage system puts a load on the pipe. A hand basin barely trickles compared to a toilet that dumps 4 to 6 litres of wastewater in a single flush. Under Australian Standard AS/NZS 3500.2, each fixture is assigned a Drainage Fixture Unit (DFU) value that reflects its discharge rate and frequency.
The total sum of DFUs connected to a drain line determines the minimum pipe size required. Sizing drains correctly prevents sluggish flow, gurgling traps, and costly sewer blockages on site.
How to Size a Drainage Pipe Using DFU Loading
To size an underground sanitary drain or horizontal branch line:
- Count every fixture connected to the drain run.
- Lookup each fixture's DFU rating from AS/NZS 3500.2 (Residential vs Commercial).
- Add up the total DFU loading.
- Compare against standard pipe size allowances (DN65, DN80, DN100, DN150, DN225).
AS/NZS 3500.2 Fixture Unit Loading Values
| Fixture Type | Residential DFU | Commercial DFU |
|---|---|---|
| WC / Toilet (Dual Flush) | 3 DFU | 4 DFU |
| WC / Toilet (Full Flush) | 4 DFU | 6 DFU |
| Basin (Hand Basin) | 1 DFU | 1 DFU |
| Bath | 3 DFU | 3 DFU |
| Shower | 2 DFU | 2 DFU |
| Kitchen Sink (Single/Double) | 3 – 4 DFU | 4 – 6 DFU |
| Laundry Trough | 3 DFU | 3 DFU |
| Washing Machine | 3 DFU | 3 DFU |
| Dishwasher | 2 DFU | 3 DFU |
| Floor Waste (50mm / 100mm) | 1 – 3 DFU | 1 – 3 DFU |
Pipe Size (DN) Maximum DFU Allowances
| Pipe Size (DN) | Max DFU (Branch Drain) | Max DFU (Building Drain) | Min Required Grade |
|---|---|---|---|
| DN65 (65mm) | 3 DFU | — | 1:40 (2.50%) |
| DN80 (80mm) | 12 DFU | — | 1:40 (2.50%) |
| DN100 (100mm) | 80 DFU | 150 DFU | 1:60 (1.67%) |
| DN150 (150mm) | 350 DFU | 700 DFU | 1:80 (1.25%) |
| DN225 (225mm) | — | 2,400 DFU | 1:100 (1.00%) |
Minimum Pipe Grade Rules Under AS/NZS 3500.2
Pipe grade (or slope) is the slope of the drain measured from the high invert to the low invert. Smaller pipes require steeper gradients because they carry less liquid volume to maintain self-cleansing velocity.
- DN100 at 1:60 (1.67% / 16.7mm per metre): The mandatory minimum gradient for standard Australian residential house drains.
- DN150 at 1:80 (1.25% / 12.5mm per metre): Used for larger commercial building drains and multi-residential main lines.
Self-Cleansing Velocity & Manning's Formula
AS/NZS 3500.2 requires sanitary drains to achieve a minimum flow velocity of 0.6 m/s at half-depth to ensure solid waste remains suspended in the flow stream. Flow velocity is calculated using Manning's equation:
V = (1 / n) × R^(2/3) × S^(1/2) Where V = velocity (m/s), n = Manning's roughness (0.009 for uPVC, 0.012 for Cast Iron), R = hydraulic radius (m), S = slope (m/m).
Real-World Worked Example: 3-Bedroom Sydney House Drain
Scenario: 3-bedroom, 2-bathroom single-storey slab home in Western Sydney with uPVC SN4 drainage over a 15-metre run.
- Fixture Tally: 2× WCs (6 DFU) + 2× Basins (2 DFU) + 1× Bath (3 DFU) + 1× Shower (2 DFU) + 1× Kitchen Sink (3 DFU) + 1× Laundry Trough (3 DFU) + 1× Washing Machine (3 DFU) + 1× Floor Waste (1 DFU) = 24 Total DFU.
- Pipe Size Selection: 24 DFU is well below 80 DFU limit → DN100 uPVC.
- Minimum Grade: 1:60 (1.67% or 16.7 mm/m).
- Total Vertical Fall: 15,000mm ÷ 60 = 250mm total fall over the 15m run.
Common Drainage Sizing Mistakes on Australian Sites
- Forgetting Floor Wastes: Omitting the floor waste DFU tally in laundry or bathroom design.
- Laying DN100 Flatter Than 1:60: Plumbing inspectors in NSW, VIC, QLD, and SA will fail drains laid flatter than 1:60 without a specific performance solution.
- Mixing Commercial and Residential Ratings: Commercial kitchen sinks require 4–6 DFU due to heavy grease loading compared to residential 3 DFU.