Drain Fall Calculator Australia — AS/NZS 3500 Pipe GradientAS/NZS 3500
Pipe details
Drain fall result
Fall details
✓ Compliant — AS/NZS 3500
⚠ Below minimum fall — AS/NZS 3500
This drain fall calculator works out total pipe fall, pipe length, or gradient for any drainage pipe to the requirements of AS/NZS 3500.2 (Sanitary Plumbing and Drainage). Fall is expressed as a ratio of 1 in X — meaning 1 mm of fall per X mm of horizontal run. The calculator performs a live compliance check against the AS/NZS 3500.2 minimum gradients: DN40–DN65 pipes require a minimum gradient of 1 in 40 (25 mm per metre); DN80 requires 1 in 60 (16.7 mm/m); DN100 requires 1 in 60; DN150 requires 1 in 150. Maximum gradients also apply — pipe flow becomes self-scouring and potentially non-compliant above 1 in 10.
Frequently Asked Questions
What is the minimum fall for a 100 mm drain in Australia?
AS/NZS 3500.2 specifies a minimum gradient of 1 in 60 for DN100 (100 mm) drainage pipes. This equals approximately 16.7 mm of fall per metre of horizontal run. A 10 m long DN100 drain must therefore fall at least 167 mm (16.7 cm) from inlet to outlet. Insufficient fall causes solids to settle in the pipe, leading to blockages.
What does a 1 in 40 fall mean?
A 1 in 40 fall means 1 mm of vertical fall for every 40 mm of horizontal run, which is equivalent to 25 mm per metre (2.5 cm/m). This is the minimum gradient required for DN50 and DN65 waste pipes under AS/NZS 3500.2. To calculate total fall: multiply the pipe length in metres by 25 mm.
Can a drain have too much fall?
Yes. Excessively steep drains — generally steeper than 1 in 10 — can cause the liquid to run faster than the solids, leaving waste deposits in the pipe. AS/NZS 3500.2 specifies maximum gradients as well as minimums. Above 1 in 10 (100 mm per metre), additional design measures are required. For most residential installations, 1 in 40 to 1 in 80 is the practical working range.
What is an invert level in drainage?
The invert level is the elevation of the lowest internal surface of the pipe — the bottom of the inside of the pipe at a given point. Drainage design works from invert to invert, ensuring continuous fall from the fixture to the sewer connection or septic tank. The invert level at the outlet of any pipe is lower than at the inlet by the total fall over that pipe run.
Who can install drainage in Australia?
Sanitary drainage work in Australia must be carried out by a licensed plumber and drainer. In most states, drainage work also requires a plumbing permit and inspection. Unlicensed drainage work is illegal and voids building insurance. Always engage a licensed plumber for any below-ground drain installation, alteration, or connection to the sewer.