Pipe size, run length and grade in, fall in millimetres, the ratio, and your end invert depth out. Minimum grades checked for every size. Free, no email.
DN100 (standard sewer drain), minimum grade 1.65% (1:61)
198mm fall
DN100 over 12m at 1.65% (1:61)
Start invert 500mm → end invert 698mm below datum.
At or above the DN100 minimum. That's 1.6mm of fall per metre, set your laser to it.
| Size | Min grade | Ratio | Fall over your 12m |
|---|---|---|---|
| DN40 | 2.5% | 1:40 | 300mm minimum |
| DN50 | 2.5% | 1:40 | 300mm minimum |
| DN65 | 2.5% | 1:40 | 300mm minimum |
| DN80 | 1.65% | 1:61 | 198mm minimum |
| DN90 | 1.65% | 1:61 | 198mm minimum |
| DN100 | 1.65% | 1:61 | 198mm minimum |
| DN150 | 1% | 1:100 | 120mm minimum |
Guide only, using the minimum grades commonly applied under AS/NZS 3500.2. Long runs, high fixture loads, pumped sections and council/authority requirements change the design. The licensed plumber and the approved plans win, not this page.
DN100 at 1:60 is 16.5mm a metre. Everyone knows it until it's 4:30pm, the trench is open, the connection point is 480mm down and you're working out whether a 14 metre run still gets there above it. That's 231mm of fall, end invert 711mm, and now you know before the excavator does.
Slide the grade and watch the invert move. The table under the result shows the minimum fall for every common size over your run, so checking an alternative is one glance rather than another round of mental arithmetic in the rain.
The minimum grades commonly applied under AS/NZS 3500.2: DN100 drains at 1.65% (about 1:60), DN65 and smaller wastes at 2.5% (1:40), DN150 at 1% (1:100). That's 16.5mm of fall per metre of DN100. The calculator flags anything below minimum for the size you pick.
Fall = length × grade. A 12 metre DN100 run at 1.65% needs 12 × 16.5 = 198mm of fall. Put your start invert depth in and the calculator gives the end invert too, which is the number you actually dig to.
The old 'water outruns the solids' rule is largely a myth at normal grades, steeper generally flushes better, and AS/NZS 3500.2 doesn't set a maximum grade for typical gravity drains. The real limits are practical: available depth at the connection point, and vertical drops needing the right fittings. What kills drains is too little fall, not too much.
No. Long runs, high fixture unit loads, boundary trap requirements, pumped sections and council or water authority conditions all change the design, and the approved plans win. This tool is for the on-site moment: you know the run and the size, you need the fall in millimetres before you set the laser.
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