Drainage Slope Calculator
Calculate the fall per metre and grade for a drainage run based on vertical drop and horizontal length.
How to use this calculator
- Enter the total vertical fall in millimetres and horizontal length in metres.
- Check the labels and units before using a result outside this page.
- Change one input at a time if you want to compare scenarios.
Formula and method
Fall per metre = fall(mm)/run(m); grade(%) = fall(mm)/(10 × run(m))
- Fall per metre = fall(mm)/run(m); grade(%) = fall(mm)/(10 × run(m))
- Suitable drainage depends on surface, flow, soil or pipe conditions, and local standards—not slope alone.
Worked examples
Worked example
A 30 mm fall over 6 m.
The fall is 5 mm/m and the grade is 0.5%.
The fall is 5 mm/m and the grade is 0.5%.
Understanding the result
Calculate the fall per metre and grade for a drainage run based on vertical drop and horizontal length.
Suitable drainage depends on surface, flow, soil or pipe conditions, and local standards—not slope alone.
Important notes
Suitable drainage depends on surface, flow, soil or pipe conditions, and local standards—not slope alone.
Displayed outputs are rounded for readability; preserve full precision when continuing a calculation.
Frequently asked questions
What does this drainage slope calculator show?
Calculate the fall per metre and grade for a drainage run based on vertical drop and horizontal length.
Which inputs should I check first?
Enter the total vertical fall in millimetres and horizontal length in metres.
Can the result be used as a final professional specification?
Suitable drainage depends on surface, flow, soil or pipe conditions, and local standards—not slope alone.
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Related explanations
Learn the method behind the number or compare the terminology before using the calculator.
- Slope — Learn what slope means and how to calculate the slope between two points.
This page provides a calculation tool plus explanatory information so you can understand what the result means and when the underlying method may not fit your situation.