How to Calculate Pipe Fall & Grade
Quick answer
Pipe fall is the difference between the upstream and downstream invert levels. Grade % = fall ÷ length × 100. The 1:X gradient is length ÷ fall — so 300 mm of fall over 30 m is 1 %, or 1:100.
The three ways drainlayers express grade
The same slope gets written three ways on drawings and consent conditions, and you need to move between them without thinking:
- —Gradient 1:X — one unit of fall per X units of run (1:100 = 10 mm per metre)
- —Grade % — fall as a percentage of length (1:100 = 1 %)
- —mm per metre — literal fall per metre of pipe (1:100 = 10 mm/m)
The calculations
Formulas
Fall = upstream IL − downstream IL
Grade % = fall ÷ length × 100
Gradient 1:X where X = length ÷ fall
Required fall for a target gradient = length ÷ X
End invert = start invert − required fall
Worked example
Worked example
Run: 42.5 m of 150 mm pipe
Upstream IL 12.840, downstream IL 12.245
Fall = 12.840 − 12.245 = 0.595 m (595 mm)
Grade = 0.595 ÷ 42.5 × 100 = 1.4 %
Gradient = 42.5 ÷ 0.595 = 1:71.4
Setting a pipe to a required gradient
Working the other way is just as common: you know the start invert and the minimum gradient, and you need the downstream invert. Required fall = length ÷ X. For 30 m at 1:100 that's 30 ÷ 100 = 0.3 m, so the end invert is 300 mm below the start.
Minimum gradients come from the drainage standard and the network operator's requirements for the pipe size and use — check the approved drawings and local code of practice rather than assuming. Laying flatter than consented is a failed inspection and a re-lay.
Pricing the whole drainage job? Fieldlink takes pipe runs from takeoff to estimate with your own laying rates.
See Drainage Estimating