Groundworks & civils calculator
Calculate pipe falls and gradients for drainage runs - a fall profile diagram shows the run stepping down to scale from manhole to manhole, and a level schedule does the setting-out maths for you.
Enter the run length and the gradient for the fall, or the length and the fall for the gradient. For whole runs chained manhole to manhole, with a level schedule and a fall profile, open the full calculator.
How far does the pipe drop over the run?
You know the two invert levels - what gradient is that?
Fall per metre and per 10 m for the gradients used most on UK sites.
| Gradient | Per metre | Per 10 m | Percent |
|---|---|---|---|
| 1 in 40 | 25 mm | 250 mm | 2.5% |
| 1 in 60 | 16.7 mm | 167 mm | 1.67% |
| 1 in 80 | 12.5 mm | 125 mm | 1.25% |
| 1 in 100 | 10 mm | 100 mm | 1.0% |
| 1 in 110 | 9.1 mm | 91 mm | 0.91% |
| 1 in 150 | 6.7 mm | 67 mm | 0.67% |
From Approved Document H (England), Table 6.
| Peak flow | Pipe | Minimum gradient |
|---|---|---|
| Under 1 l/s | 75 mm | 1 in 40 |
| Under 1 l/s | 100 mm | 1 in 40 |
| Over 1 l/s | 75 mm | 1 in 80 |
| Over 1 l/s | 100 mm | 1 in 80 (at least 1 WC) |
| Over 1 l/s | 150 mm | 1 in 150 (at least 5 WCs) |
A design aid only - check the current Approved Document H (or the Scottish, Welsh or Northern Ireland equivalent), the drainage design and the local water company's requirements before you dig. Very steep falls can be a problem too, as liquids outrun solids.
The Runs tab's run list and fall profile diagram, the Levels tab's level schedule, and the Guide tab's minimum fall reference - all shown in dark mode.
Runs tab - run list and fall profile (dark mode)
Levels tab - level schedule (dark mode)
Guide tab - minimum fall reference (dark mode)
A longitudinal fall profile diagram chains every run end to end by cumulative chainage and invert level, stepping downward to scale - a genuine picture of the run, not just a table of numbers.
Add a run for each length of pipe - each one's start level is automatically the previous run's end level, chained from a single start invert level you set.
Chainage, start level, end level and fall for every run, laid out as a setting-out schedule - ready to check against a drainage layout drawing or hand to groundworkers.
Each run is checked against a minimum self-cleansing fall for its pipe diameter and drainage type - runs flatter than recommended are flagged right away, on the run list and on the profile diagram itself.
Three pipe diameters, each with its own minimum fall for foul and for surface water drainage - switch per run, no lookup table required.
Copy a plain-text summary or export it as a standalone HTML file. Light/dark theme remembers your choice, and rotating sample runs let you explore the app.
MicroDrainage (Innovyze, an Autodesk company) is the well-known industry-standard software for detailed drainage and SuDS network design used by civil engineers - here's how a focused, free contractor fall-check tool compares.
| Feature | Drainage Fall Calculator | MicroDrainage |
|---|---|---|
| Pricing model | Free - no account, no subscription | Enterprise subscription, custom quote (rated high-cost by reviewers) |
| Works fully offline | ✓ | ~ |
| Instant standalone fall/level sense-check | ✓ | ✕ |
| Full hydraulic network modelling & SuDS compliance reporting | ✕ | ✓ |
| CAD (Civil 3D) integration | ✕ | ✓ |
| Minimum-fall check flagged automatically in minutes | ✓ | ~ |
| Built for site managers/contractors rather than drainage engineers | ✓ | ✕ |
| Requires specialist training | ✕ | ✓ |
Comparison based on Innovyze/Autodesk's publicly listed product information as of August 2026 (innovyze.com, g2.com, alumascwms.co.uk) - features and availability change over time, so check innovyze.com for current details. MicroDrainage is a trademark of its respective owner; this comparison is for informational purposes and isn't sponsored by or affiliated with Innovyze or Autodesk.
Three runs chained end to end from a known start invert, each with its own gradient and pipe size, checked against the minimum fall for its diameter.
Larger pipes can run flatter, which is why the 225 mm outfall run is fine at 1 in 150 while the 100 mm branch is set at 1 in 80. Change that last run to 1 in 300 and it turns amber as shallower than recommended - the check you want before the trench is dug. Open the browser edition to try it with your own figures.
No - use it straight from your browser, on a phone, tablet or PC, with no sign-up. It works offline once it has loaded, and you can add it to your home screen or desktop like an app. There is also a Windows desktop version in the Microsoft Store if you prefer an installed app.
No. Drainage Fall Calculator opens straight away with no sign-in - install it from the Microsoft Store and you're ready. Registering your copy for update notices is optional.
Each run's start invert level is the previous run's end level, chained from a single start level you set - the same way a drainage run is set out on site from manhole to manhole.
Approximate self-cleansing minimum fall figures by pipe diameter and drainage type (foul or surface water), broadly following UK Building Regulations Approved Document H guidance. It's a design aid, not a substitute for your drainage engineer's design - always confirm against the current regulations.
Approved Document H gives a minimum of 1 in 80 for a 100 mm foul drain carrying a peak flow over 1 l/s and serving at least one WC, and 1 in 40 for lower flows. Many groundworkers simply lay 100 mm foul drains at around 1 in 40 to 1 in 80. Surface water pipes can usually run a little flatter.
Divide 1,000 mm by the gradient. 1 in 40 is 25 mm per metre, 1 in 80 is 12.5 mm per metre and 1 in 100 is 10 mm per metre. Multiply by the run length for the total fall - 12 m at 1 in 80 falls 150 mm.
1 in 40 is 2.5%. To convert any gradient, divide 100 by the second number: 1 in 80 is 1.25% and 1 in 100 is 1%.
