Drainage Fall Gradient Calculator

How it is calculated

A gradient written as "1 in X" means the pipe drops 1 unit for every X units of horizontal run. The tool takes the run length L in metres and the denominator X, and returns the total fall F = L ÷ X in metres, then multiplies by 1000 for millimetres (shown to one decimal). It also gives the same slope as a percentage, 100 ÷ X, and as millimetres of fall per metre of run, 1000 ÷ X. The run must be above zero and X must be at least 1, because a fall larger than the run itself is impossible; an X below 1 (such as 0.5) shows no result, and both fields are required. A larger X means a gentler slope, so 1 in 110 falls less than 1 in 40. The tool does not check whether the gradient suits your pipe diameter or the local rules, which differ between countries, and it measures run along the horizontal, not along the sloping pipe.

Common mistakes

• Entering the gradient as a percentage, for example 2.5, instead of the denominator, 40. A value of 2.5 is read as 1 in 2.5, a 40% slope that gives a huge fall (4,800 mm over 12 m).
• Typing a gradient below 1, such as 0.5, thinking of a fraction. The field is the X of "1 in X", so values under 1 are rejected and show no result.
• Typing the run in millimetres or feet. The field is metres, so 12000 for a 12 m run gives a fall a thousand times too large.
• Mixing up the direction of the gradient. A bigger number such as 1 in 110 is flatter than 1 in 40, so use the smaller denominator when you want more fall.
• Using the fall as an absolute level. The result is only the drop between the two ends, so add it to the starting invert level yourself.