Plumbing Fall Formulas
Total Fall: Pipe Length × Fall per Metre
Grade Percentage: 100 ÷ Grade Ratio
Note: Grades shown are minimum requirements per AS/NZS 3500. Always check local council regulations.
Plumbing Fall Calculator — Pipe Fall & Grade (AS/NZS 3500)
Every drain in Australia must maintain minimum fall for self-cleansing velocity under AS/NZS 3500.2. Too little fall causes blockages; too much creates turbulence. This calculator determines the correct fall, grade ratio, and invert levels for any pipe run.
Minimum Fall Requirements (AS/NZS 3500)
| Application | Pipe Size | Min Grade | Fall/m |
|---|---|---|---|
| Basin/Sink Waste | 40–50mm | 1:40 | 25mm |
| Sanitary Drain | 65–100mm | 1:60 | 17mm |
| Sanitary Drain | 150mm | 1:80 | 12.5mm |
| Stormwater | 90–100mm | 1:100 | 10mm |
| Subsoil/Ag Drain | 100mm | 1:200 | 5mm |
Formulas
- Fall per metre = 1000 / Grade Ratio
- Total fall = Length (m) × Fall/m
- Percentage = 100 / Grade Ratio
Frequently Asked Questions
Minimum fall for 100mm sewer?
1:60 under AS/NZS 3500.2 = 16.7mm per metre. A 10m run needs 167mm total fall.
What does 1:60 fall mean?
The pipe drops 1mm for every 60mm of run, or 16.7mm per metre. Equivalent to 1.67% slope.
Minimum fall for stormwater?
1:100 for 90–100mm pipes (10mm/m). Larger pipes can use 1:150 to 1:200.
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