Basic Drainage Grade Calculator
A free, browser-based calculator. Runs entirely in your browser — no sign up, nothing stored.
Enter Values
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.
Related tools
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.