Drain Pipe Slope Calculator

Common Australian Plumbing Gradients

While specific requirements can vary based on the AS/NZS 3500.2 Sanitary Plumbing and Drainage standard, common gradients include:

• 1:40 (2.50%) - Often used for smaller pipes (DN40 / DN50) or unvented branch drains to ensure rapid clearing.
• 1:60 (1.65%) - The standard gradient for most main drains (DN100) and general sanitary plumbing. This balance ensures water moves fast enough to carry solids but not so fast that it leaves solids behind.
• 1:80 to 1:100 - Sometimes permitted for larger civil drainage or where vertical space is strictly limited, subject to hydraulic design verification.

How to Use This Calculator

1. Select your Gradient: Choose the ratio that matches your pipe size or design requirement (e.g., 1:60 for a standard DN100 drain).

2. Enter Length: Input the total length of the pipe run in metres.

3. Calculate: The tool will display the total vertical drop (fall) required in millimetres. This is the difference in height needed between the start and end of the pipe.

Metric vs Imperial

Unlike the US system which uses "inches per foot," Australian plumbing relies on metric ratios. If you are using a digital level, you can often switch it to display % grade (e.g., set to 1.65% for a 1:60 slope).

Why Correct Fall Matters

• Too Flat: Solids will settle in the pipe, causing eventual blockages.
• Too Steep: Liquids may flow faster than solids (scouring), leaving waste behind to dry out and block the pipe.
• Backfall: If pipes slope the wrong way, water pools, creating health hazards and immediate blockages.