Calculate the correct fall for internal waste pipes and external drainage runs in the UK, US and Australia. Use it for quick site checks, trench planning, waste pipe runs, reverse fall checks and simple level setting before you start cutting, boxing in or digging.
Choose your country, select internal or external, then use either the standard calculator or the reverse check if your site conditions are already fixed.
| Distance | Cumulative drop | Depth at this point |
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This section explains the preset you are using and gives a practical site check.
Using a practical branch waste preset.
Length looks sensible for the selected preset.
This is a planning and checking tool. Final compliance still depends on the full layout, fittings, venting, inspection requirements and local rules.
Use this when you know the run length and want the correct fall, percentage and site setting-out figures.
Use this when the site is already dictating the fall. Enter the run length and available drop to see what ratio you actually have.
For external drainage, add a start depth or cover and the tool will estimate the end depth needed to hold the selected fall.
The cumulative table helps you keep a trench or waste run honest as you go, rather than only checking the start and finish.
The reverse check is ideal when ceiling height, joists, floors or existing drainage are already limiting what you can do.
Use the copy summary button to send the key numbers to a client, a colleague or your own phone without messing around with screenshots.
This page is designed to help with waste pipe planning, simple foul drainage runs, trench set-out, site checks and quick fall comparisons across the UK, US and Australia.
It is especially useful when you want to compare a standard rule with real site conditions before you start cutting floors, drilling walls or digging out runs.
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A 1:40 fall means the pipe drops 1 unit vertically for every 40 units horizontally. Over 4 metres, that works out to 100mm of fall.
The calculator defaults to the units people in that country are more likely to use on site. UK and Australian users normally think in metres and millimetres, while US users commonly think in feet and inches.
It helps when the job is already constrained. If you only have a fixed amount of fall available, the reverse check tells you what ratio that gives you so you can judge whether it is sensible for the job.
Yes. Add a start depth or cover and the calculator will show the estimated end depth needed to maintain the selected fall across the run.
Not necessarily. Too little fall is a common problem, but a run that is much steeper than it needs to be is not automatically the best answer either. The aim is to get the right fall for the job.
