Geometry & drainage
Drainage fall for a chosen gradient
- where
- fall (m; the calculator shows mm)
- horizontal run (m)
- gradient denominator (1 in N)
Explain this equation
Works out how much a drain or waste pipe must fall over a given run at a chosen gradient. Use it to set levels before laying pipe.
L is the horizontal run. N is the gradient denominator: 1 in N means one unit of fall per N units of run. h is the fall needed.
Fall is run divided by N. The calculator works in metres and shows mm.
Sense check: 6 m at 1 in 80 needs 75 mm. At 1 in 40 the fall is 25 mm per metre; at 1 in 80 it is 12.5 mm per metre.
Watch out: this is geometry only. Choose the gradient from the applicable guidance, which also depends on pipe size and flow. Rules differ between UK nations.
Enter your values to see the result.
Good to know
Enter an independently selected gradient. This tool does not determine the permitted gradient, pipe diameter, discharge units or self-cleansing performance. England guidance differs from other UK nations.
Sources
Results are arithmetic only. Worked examples are not recommended design settings. This library does not provide automatic gas pipe selection, live gas tightness tests, purging procedures, combustion acceptance limits, installation approval or commissioning certification. Manufacturer data and the applicable current method remain essential.