The one that gets written backwards
Grades are written as 1-in-something. The trap is that the bigger the second number, the flatter the pipe. 1:40 drops 25mm every metre. 1:60 drops 16.7mm. 1:100 drops 10mm. So 1:40 is steeper than 1:60, even though 40 is the smaller number.
Get that backwards on a job and you lay a drain flatter than it should be, it fails to self-cleanse, and the callback arrives about eighteen months later with solids in it. This calculator does the comparison the right way round and says so out loud when your grade is the flatter one.
Minimum grades by pipe size
AS/NZS 3500.2 sets minimum grades for sanitary drainage, and they get shallower as the pipe gets bigger — a bigger bore carries the same flow at a lower slope. Described in the units the trade actually uses:
| Pipe size | Minimum grade | As a percentage | Fall per metre |
|---|---|---|---|
| DN40 — basin, sink waste | 1 in 40 | 2.500% | 25.0 mm |
| DN50 — combined waste | 1 in 40 | 2.500% | 25.0 mm |
| DN65 | 1 in 40 | 2.500% | 25.0 mm |
| DN80 | 1 in 40 | 2.500% | 25.0 mm |
| DN100 — sewer / soil drain | 1 in 60 | 1.667% | 16.7 mm |
| DN150 | 1 in 80 | 1.250% | 12.5 mm |
These follow published Australian guidance on the standard's drainage provisions. Your local water authority can be stricter, and a few are, so check theirs before you dig.
A worked example
Ten metres of DN100 soil drain from the house to the boundary, laid at its minimum grade:
- Minimum grade for DN100 is 1 in 60
- Fall = 10 × 1000 ÷ 60 = 166.7 mm over the whole run
- That is 16.7mm every metre — about the thickness of two 10-cent pieces per metre, which is why you set it with an instrument and not by eye
- Start invert 1000mm below your datum peg, so the far end sits at 1000 − 166.7 = 833.3 mm
Stretch that to a 24m run and the fall becomes 400mm. That is the number that decides whether the drain still clears the connection point at the far end, and it is worth working out before the excavator turns up rather than after.
Too steep is a problem too
On a waste line, steeper is mostly fine. On a soil drain carrying solids, it is not. Run a DN100 sewer at 1:10 and the water sprints away and leaves the solids behind — the drain self-blocks from the top end down, and nobody believes you when you tell them the fall is the cause.
A drain wants enough velocity to scour and not so much that the liquid separates from the load. Where the ground drops away faster than the pipe should, you step it down at a junction or a riser rather than tipping the whole run.
Setting it out
- Rotating laser and a staff — fastest on anything over a few metres. Set the grade on the laser or read the difference off the staff at each end.
- Level and a grade gauge — a spirit level with a fall gauge, or a packer of known thickness under one end. A 1200mm level with a 20mm packer is 1:60.
- String line and boning rods — old school, still accurate, and it works in a trench where a laser gets in the way.
- Bed the pipe evenly. A drain that is on grade at both ends and sagging in the middle is a belly, and it holds water and traps solids exactly like a flat drain does.
Invert levels, plainly
The invert is the inside bottom of the pipe, not the top and not the outside. Every level on a drainage plan is an invert, and mixing invert with obvert or with finished surface is how a run ends up a pipe-wall thickness out at each junction. Pick one datum — a peg, a step, a benchmark — and take every reading off that same one.
Working upstream from a fixed connection point is usually safer than working downstream from the house. The connection is the level you cannot change; the fixture end usually has some give.
Quoting drainage
Fall drives depth, and depth drives everything expensive: excavation, shoring, spoil removal, bedding sand and reinstatement. A long run at a shallow minimum grade is a cheaper dig than the same run laid steeper. Work the fall out first, then price the hole.
Drainage is licensed plumbing work in every Australian state and territory. Use this to estimate and set out; the inspection, the connection and the paperwork stay with the licensed plumber doing the job.
Set the grade. Now price the dig.
Yamate turns metres of trench, pipe and hours into a quote the customer accepts on their phone, then an invoice, then a payment — with GST handled and the chasing done for you.
Try Yamate free → Free to try · no card · sign in with a link, no password to rememberPipe fall & drainage grade FAQ
What is the minimum fall for a 100mm sewer pipe in Australia?
A grade of 1 in 60, which is 1.667% or 16.7mm of fall per metre. Over a 10 metre run that is 166.7mm. Confirm against AS/NZS 3500.2 and your local water authority, which can be stricter.
Is 1:40 steeper or flatter than 1:60?
Steeper. 1:40 drops 25mm per metre, 1:60 drops 16.7mm. The bigger the number after the colon, the flatter the pipe — which is the mistake that passes a drain laid too shallow.
What is the minimum fall for a 40mm or 50mm waste pipe?
1 in 40, which is 2.5% or 25mm of fall per metre. Small waste pipes need a steeper grade than a big drain because there is less bore to carry the flow.
Can a drain have too much fall?
Yes, on anything carrying solids. Too steep and the water runs ahead of the solids and leaves them in the pipe, so the drain blocks itself. Step the run down at a junction instead of tipping the whole length.
What does invert level mean?
The inside bottom of the pipe. Drainage plans use invert levels, so measure to the inside of the bottom of the bore, not the top of the pipe and not the outside of it.
How do I set out a fall on site?
A rotating laser and staff is quickest. A level with a fall gauge works too — a 1200mm level with a 20mm packer under one end is 1:60. String line and boning rods still work in a tight trench.
Does 1% fall meet the minimum for a soil drain?
1% is 1 in 100, which is flatter than the 1 in 60 minimum for DN100 — so no. Enter 100 into the grade field above and the calculator will flag it for you.
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General information for Australian trades, and an estimate only — not tax, financial or legal advice. It is not engineering, building or design advice either: a calculator works out a quantity from the figures you enter, and anything that needs sign-off still needs your engineer, certifier or building surveyor. Minimum grades follow published Australian guidance on the AS/NZS 3500.2 drainage provisions and can change between editions. Local water authorities and councils may impose stricter requirements. Sanitary drainage is licensed plumbing work in every state and territory — this calculator is a setting-out and estimating aid, not a design, an approval or a substitute for the standard itself.