Free drain fall calculator
Set out a drain with peg levels and laser readings along the run, or check whether it will fall from the house to the sewer connection. Grades as 1:60, % or mm per metre, with the usual minimum grades for each pipe size.
Work out a drain’s fall
No sign-up. Works offline on site, nothing is uploaded, and the levels are ready to peg out.
Open the calculatorOpens in a new tab. Works on a phone, tablet or computer.
What it does
- Set out a drain: add each straight leg with its length, grade and pipe size, and get the fall on each leg and the total.
- Peg levels and laser readings: every 0.5, 1, 2 or 5 m, plus every bend or junction, as fall from the start, invert RL or laser staff reading.
- A long section: a side view of the run with the grade and fall on each leg.
- Will it fall? From two laser readings or levels and the length, the grade available, whether it meets the minimum, how much more fall you’d need, and how far it would reach at the minimum grade.
- Grades any way you write them: 1 in 60, 1.67% or 16.7 mm per metre, switching between them without losing the grade.
- Minimum grade checks: flags any leg flatter than the usual minimum for its pipe size.
Usual minimum grades for sanitary drains
| Pipe size | Minimum grade | Fall per metre |
|---|---|---|
| DN65 and smaller | 1:40 (2.5%) | 25 mm |
| DN80 and DN100 | 1:60 (about 1.65%) | 16.7 mm |
| DN125 | 1:80 (1.25%) | 12.5 mm |
| DN150 | 1:100 (1%) | 10 mm |
From AS/NZS 3500.2. Some fixtures, authorities and site conditions need steeper grades. Stormwater grades come from AS/NZS 3500.3, the plans and the local council.
Grade conversions
| Ratio | Percentage | mm per metre | Fall over 10 m |
|---|---|---|---|
| 1:40 | 2.5% | 25 mm | 250 mm |
| 1:60 | 1.67% | 16.7 mm | 167 mm |
| 1:80 | 1.25% | 12.5 mm | 125 mm |
| 1:100 | 1% | 10 mm | 100 mm |
| 1:200 | 0.5% | 5 mm | 50 mm |
Common questions
How much fall does a 1:60 grade give?
1:60 is about 16.7 mm of fall per metre, or 167 mm over 10 m. 1:40 is 25 mm per metre, 1:80 is 12.5 mm and 1:100 is 10 mm.
What is the minimum grade for a 100 mm sewer drain?
The usual minimum for a DN100 sanitary drain under AS/NZS 3500.2 is 1:60 (about 1.65%). DN65 and smaller fixture pipes usually need 1:40, DN125 1:80 and DN150 1:100. Some fixtures, authorities and site conditions need more, so check the approved plans.
How do I check a drain will fall to the sewer connection?
Take a laser staff reading (or level) to the invert at the house end and at the connection point, and measure the length along the route. The “Will it fall?” tab gives the grade available, whether it meets the minimum for the pipe size, and how much more fall you’d need if it doesn’t.
How do I use it with a laser level?
Enter the staff reading to the invert at the start. The peg table then gives the reading you should see at every peg along the run. Readings get bigger as the drain goes down.
Can a drain be too steep?
Very steep drains can leave solids behind as the water runs ahead, so plumbers keep grades sensible and use vertical drops where the ground falls away. The calculator warns about anything steeper than 1:5.
Is anything uploaded?
No. It all runs in your browser, works offline once open, and keeps your last drain on your device.
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