Calculating pipe fall and gradient

Core idea

A gradient written 1:N means one unit of vertical fall for every N units of horizontal run. Use the same units before calculating. Fall equals horizontal run divided by N. Gradient denominator equals run divided by fall. State the direction: the pipe falls towards the downstream point.

What to recognise

• Convert metres to millimetres before dividing when the answer is needed in millimetres.
• Use horizontal run rather than sloping pipe length for the basic gradient calculation.
• For 1:40 over 6 m, calculate 6000 ÷ 40 = 150 mm fall.
• Check the result against the available levels and required cover.
• A steeper numerical fall has a smaller 1:N denominator, so 1:40 is steeper than 1:80.

Apply it

Start with a labelled sketch showing upstream level, downstream level and horizontal run. Calculate the required fall, subtract it in the direction of flow, then compare with measured invert levels. Recheck units before setting out clips or excavation depths.

Keep the boundary clear

The arithmetic does not establish that a gradient, diameter or arrangement complies. Use the applicable drainage design guidance for capacity, self-cleansing, branch geometry, ventilation, access and bedding. Excessive gradient can also create performance problems; “steeper is always better” is not a design rule.