Drainage Fall Calculator

This drainage fall calculator works out the right gradient for a foul or surface water drain, converts between fall formats, and checks the self-cleansing velocity — the three things Building Control will expect a drainage layout to get right. Enter the pipe diameter and the peak flow and it returns the minimum recommended gradient, following Approved Document H of the Building Regulations and the Colebrook–White equation. Treat it as a fast first pass for sizing and checking a drain run.

How to use the drainage fall calculator

Choose whether the pipe is carrying foul or surface water, enter the internal pipe diameter and the peak flow it has to take, and the tool returns the minimum recommended gradient, the full-bore velocity, and the total fall over a run length you specify. The fall converter turns any gradient into a 1-in-X ratio, millimetres per metre, or a percentage, so the figure matches whatever your drawings or Building Control officer use.

What gradient should a drain be?

For most domestic foul drains, Approved Document H sets a minimum gradient of between 1:40 and 1:150 depending on the pipe size and the flow it carries. A 100 mm foul drain serving at least one WC should fall at 1:80; a 150 mm drain needs to be laid at 1:150 and serve at least five WCs to stay self-cleansing. Surface water pipes follow the same hydraulic logic but are sized for the design storm rather than appliance flows.

In practical terms, 1:80 is a fall of 12.5 mm for every metre of pipe. The table below summarises the Approved Document H starting points.

Minimum fall for foul drainage (Approved Document H)

The minimum fall for foul drainage comes from Approved Document H1 (Table 6 and Diagram 8). The thresholds exist to keep the flow self-cleansing: too shallow and solids settle out and block the pipe; the 1 l/s peak-flow threshold is what allows a 100 mm drain to be laid at the gentler 1:80 rather than steeper. The calculator applies these rules automatically once you enter the pipe size and flow.

Self-cleansing velocity — why the gradient matters

A drain has to flow fast enough at peak to carry solids along rather than letting them settle. The calculator reports the full-bore velocity for your pipe and flow using the Barr (1975) approximation to the Colebrook–White equation, so you can confirm the gradient you have chosen keeps the pipe self-cleansing. If the velocity comes out low, increase the gradient or check the pipe is not oversized for the flow.

Converting between fall, gradient and percentage

Drainage falls get written three different ways and it is easy to mix them up. A 1:80 fall is the same as 12.5 mm per metre, which is the same as 1.25%. The converter moves between all three and works out the total vertical drop over a given run, which is the figure you need when you are setting invert levels on a long drain.

Further reading: foul, surface and combined sewers explained · what drainage information is needed for planning permission · keeping drains clear to prevent surface water flooding · the full set of Approved Documents.