Drainage fall from length & ratio

Drainage runs are specified as a gradient ratio — 1:40, 1:80, 1:110 and so on — but a spirit level and a story pole want a figure in mm, not a ratio. This tool takes the length of a pipe run and the gradient you're working to, and gives you the total fall over that run in millimetres, along with the fall per metre and the equivalent percentage, so you can set out levels without reaching for a calculator halfway down a trench.

Reading a gradient ratio

A fall of 1:80 means the pipe drops 1mm for every 80mm it runs — so over a metre (1000mm) that's 1000 ÷ 80 = 12.5mm of fall. Multiply that per-metre figure by the total run length and you've got the total drop from the high end to the low end of the pipe. Shallower ratios (1:110, 1:150) are common on longer soil runs where a steeper fall would put the invert too deep by the time it reaches the manhole; steeper ratios (1:40, 1:60) are typical on shorter above-ground waste runs where self-cleansing velocity matters more than depth.

Getting the fall right matters for two reasons that pull in opposite directions: too shallow and solids settle out instead of being carried along; too steep and the liquid outruns the solids, leaving them behind. That's why Building Regs give a range rather than a single figure for each pipe diameter, rather than "steeper is always better."

A worked example

An 8m run of 110mm soil pipe laid to 1:80 gives a fall per metre of 12.5mm, so the total fall over 8m is 100mm — call it a 10cm drop from one end of the run to the other. Lay the same 8m run to a shallower 1:150 instead and the total fall drops to 53.3mm, which matters if you're tight on invert depth at the connection end.

What this isn't

This does the arithmetic for a ratio and a length — it doesn't check the result against Building Regs Part H, which sets minimum and maximum falls depending on the pipe diameter (foul drainage in particular has both a floor and a ceiling on gradient, not just a minimum). It also doesn't account for changes in level along the run, bends, or the invert depth you're starting from — those still need working out on the drawing or on site before you cut a single length of pipe.