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Fall per Metre: 1000 ÷ Grade Ratio (e.g. 1:40 = 25 mm/m) 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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