Drainage fall calculator
PlumberMate AU · Australian trade tools
Convert a chosen pipe gradient into the vertical fall over a measured run. Use metric units for Australian job planning.
Calculated fall
166.7 mm
1.67% · 16.7 mm per metre
How pipe fall is calculated
Fall (mm) = run length (m) × 1,000 ÷ gradient ratio. A 1:60 ratio means one unit of vertical fall for every 60 units of horizontal run.
Worked example: 10 m at 1:60 gives 166.7 mm of fall, equivalent to 1.67% or 16.7 mm/m. This does not determine the required grade.
Assumptions to check on site
The run is horizontal distance and the ratio is supplied by you. This conversion does not assess capacity, pipe sizing, obstructions, fittings or the required minimum grade.
Australian plumbing FAQs
How do I work out the fall for a 10 m drain at 1:60?
Multiply the run length in metres by 1,000 and divide by the gradient ratio: 10 × 1,000 ÷ 60 = 166.7 mm of fall. That is the same fall as 1.67% or 16.7 mm per metre.
What does a 1:60 gradient actually mean?
One unit of vertical drop for every 60 units of horizontal run. The same slope can be written as a ratio, a percentage or millimetres per metre, and the calculator shows all three so you can check a string line or a level reading either way.
Does this tell me the minimum grade I am allowed to use?
No. Required grades depend on pipe size, the application and the authority that approves the work. Check the applicable Australian standard, the manufacturer’s installation instructions and your local water or plumbing authority before you dig.
Does the required fall change with pipe diameter?
This tool only converts a ratio into a fall — it does not size pipework. Capacity and self-cleansing behaviour depend on diameter, flow, slope and pipe condition, which need a separate check against the applicable requirements.
Why does my measured fall differ from the calculated fall?
Common causes are measuring to the wrong reference point, sight-line or level error over a long run, excavation depth not matching pipe invert depth, and ground settlement. Re-measure at both ends against a fixed level and compare with the design invert.
Educational and estimating aids only. For plumbing, drainage or gas work, verify assumptions against current Australian standards, manufacturer instructions and local requirements. These tools do not replace professional assessment.