Pipe Gradient and Fall Calculator

For a 1:N gradient, total fall = pipe length ÷ N (using the same units). Example: a 40 m run at 1:40 falls 1.0 m (1000 mm). Ending invert = starting invert − total fall on a gravity drain.

A 1:40 gradient means the pipe falls 1 unit vertically for every 40 units of length, about 25 mm per metre. 1:80 is shallower (12.5 mm per metre). Steeper ratios (smaller N) drain more aggressively; always check the minimum gradient for the pipe diameter and foul or surface water use.

Measure the pipe length between the points, note the starting and ending invert levels, then total fall = start − end. The ratio N = length ÷ fall. This calculator does that conversion and also reports fall per metre and gradient as a percentage.

UK drainage drawings usually show invert levels in metres to three decimal places (e.g. 52.450 m AOD). Falls along short runs are often discussed in millimetres. This tool accepts either and always shows total fall in both mm and m.

Fall per metre is the drop for each metre of pipe: fall ÷ length. A 1:40 run has 25 mm/m; 1:80 has 12.5 mm/m. It is a handy site check when setting laser levels or string lines.

That is an adverse (rising) gradient for gravity drainage. The calculator still reports the ratio and warns you, useful for checking survey data, but a foul or surface water gravity pipe should fall in the direction of flow unless it is a pumped rising main.