Basic Drainage Grade Calculator
Computes drainage pipe fall and grade from invert levels against minimum self-cleaning slopes.
Enter Values
Minimum 0
How to use this calculator
- Enter the upstream and downstream invert reduced levels and the pipe length.
- Read the fall, the grade as a percentage and as 1 in N.
- Optionally enter a minimum grade (1:N) to check the pipe is steep enough.
How it works
The grade is the fall divided by the length: grade = (upstream invert − downstream invert) ÷ length, shown as a percentage and as a 1 in N ratio.
A minimum (self-cleaning) grade keeps the flow fast enough to carry solids. The tool compares your grade with the minimum you enter — use your water authority's required value.
Worked example
Invert 100.500 to 100.000 over 100 m. Fall = 0.500 m, grade = 0.5% = 1 in 200. Against a 1:200 minimum it just meets the requirement.
Frequently asked questions
What's a typical minimum drainage grade?
It depends on the pipe size, material and authority — commonly around 1:100 to 1:200 for small sewers, flatter for larger pipes. Always use the value your authority specifies; this tool is a guide only.
Why does the grade need a minimum?
Too flat and the flow slows down, letting solids settle and block the pipe. The minimum 'self-cleaning' grade keeps the velocity high enough to keep it clear.
Is this official course material?
No. It is free study support mapped to surveying course levels — not official North Metropolitan TAFE content or advice. Always follow your lecturer and the official assessment brief, and check your own working.
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Cite this tool
Zerdly. (2026). Basic Drainage Grade Calculator. Retrieved from https://www.zerdly.com/tools/basic-drainage-grade-calculator/
Tip: Enter any known values to calculate the remaining results.
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