Drainage Fall & Pipe Sizing — AS/NZS 3500.2 Quick Reference for Plumbers
Bookmark This: These are the drainage tables you look up most often on site. Minimum falls, maximum falls, pipe sizing by fixture units, and branch drain requirements — all from AS/NZS 3500.2. For instant lookups on your phone, use the free standards tool.
Minimum & Maximum Drainage Grades
All sanitary drainage pipes must be laid to a grade (fall) that ensures self-cleansing velocity without excessive turbulence. The minimum grade depends on pipe diameter:
| Nominal Diameter | Minimum Grade | As Percentage | Fall per Metre | Maximum Grade |
|---|---|---|---|---|
| DN 40 | 1 in 40 | 2.5% | 25mm per metre | 1 in 10 (10%) |
| DN 50 | 1 in 40 | 2.5% | 25mm per metre | 1 in 10 (10%) |
| DN 65 | 1 in 40 | 2.5% | 25mm per metre | 1 in 10 (10%) |
| DN 80 | 1 in 60 | 1.65% | 16.5mm per metre | 1 in 10 (10%) |
| DN 100 | 1 in 60 | 1.65% | 16.5mm per metre | 1 in 10 (10%) |
| DN 150 | 1 in 60 | 1.65% | 16.5mm per metre | 1 in 10 (10%) |
| DN 225+ | 1 in 90 | 1.1% | 11mm per metre | 1 in 10 (10%) |
Fixture Unit Ratings (Drainage)
Each plumbing fixture contributes a "fixture unit" (FU) load to the drainage system. Pipe sizing is based on total fixture units:
| Fixture | Fixture Units | Minimum Trap/Waste Size |
|---|---|---|
| Basin (hand wash) | 1 | DN 40 |
| Shower | 2 | DN 50 |
| Bath | 3 | DN 50 |
| Kitchen sink | 3 | DN 50 |
| Washing machine | 3 | DN 50 |
| Toilet (WC) | 4 | DN 100 |
| Floor waste | 1 | DN 50 (or DN 80 if combined) |
| Dishwasher | 2 | DN 40 |
Drain Pipe Sizing by Fixture Units
| Pipe Diameter | Max Fixture Units (Branch) | Max Fixture Units (Main Drain) | Typical Use |
|---|---|---|---|
| DN 50 | 6 | — | Single bathroom (basin + shower) |
| DN 65 | 12 | — | Multiple fixtures without WC |
| DN 100 | 50 | 150 | Standard residential (with WC) |
| DN 150 | 200 | 600 | Multi-unit residential, commercial |
Practical Example: Standard 3-Bedroom House
A typical 3-bedroom house with 1 bathroom, 1 ensuite, kitchen, and laundry:
- Main bathroom: WC (4) + basin (1) + shower (2) = 7 FU
- Ensuite: WC (4) + basin (1) + shower (2) = 7 FU
- Kitchen: sink (3) + dishwasher (2) = 5 FU
- Laundry: washing machine (3) + tub (3) = 6 FU
- Total: 25 FU → DN 100 main drain at 1:60 minimum grade
Quick Calculator
Don't want to add fixture units manually? TradeDesk's free pipe sizing calculator does this automatically — input your fixtures, get the pipe size and grade instantly with an isometric diagram.
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