Drainage Fall Calculator
Total fall over a pipe run for a given gradient, in millimetres, percent and mm per metre.
Results
Fall
200.0
mm
Fall
0.2000
m
Slope
1.667
%
Fall
16.67
mm/m
What this tool does
Gradients on drawings are written as a ratio — 1 in 60, 1 in 80 — while the person setting out on site works in millimetres per metre. This converts between the two and tells you exactly how far the invert has to drop from one end to the other.
Formula
fall = length × slope
Variables
| Symbol | Meaning | Unit |
|---|---|---|
l | Pipe length | m |
gd | Gradient (1 in …) | — |
F | Fall | mm |
FM | Fall | m |
P | Slope | % |
MM | Fall | mm/m |
Worked example
- Pipe length12 m
- Gradient (1 in …)60
- Fall200.0 mm
- Fall0.2000 m
- Slope1.667 %
- Fall16.67 mm/m
Limitations
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
Frequently asked questions
Can a drain be too steep?
Yes. If the water runs away faster than the solids it carries, the solids are left behind and the pipe eventually blocks. Foul drainage is usually laid between about 1:40 and 1:110 for exactly this reason: fast enough to be self-cleansing, slow enough to carry its load. The applicable range depends on the pipe diameter and the local code.