How the calculation works
Geometric level relationship with metres converted to millimetres.
Formula
Fall = length ÷ gradient; gradient = length ÷ fall
Worked example
Length10 m
Gradient1:80
Expected fall125 mm
End level from 10.000 m9.875 m
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Drain fall, gradient and level calculator
Calculate fall from length and gradient, gradient from length and fall, or an end level from a starting level.
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Calculation result
Formula
Assumptions and limitations
Assumptions
- The gradient is uniform over the entered length.
- Levels use the same datum.
What this does not check
- Does not select pipe size or minimum gradient.
- Does not verify drainage standards, discharge units, access, ventilation or hydraulic capacity.
Formula version: 1.0 · Last reviewed: 16 August 2026
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