Calculate pipe falls and gradients for drainage runs

Drainage runs

Run Length (m) Fall (1 in) Pipe Type WCs Fall (m)
Runs chain end to end from the start invert level - each run's start level is the previous run's end level. Fall is expressed as "1 in N" (a fall of 1m over N metres of run) - larger N means a flatter, shallower fall.
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Total length (m)
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Total fall (m)
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End level (m)
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Shallow runs

Fall profile

Invert level vs cumulative chainage, to scale
Meets minimum fall Shallower than recommended

Level schedule

RunChainage fromChainage toStart levelEnd levelFall
A set-out schedule for the drainage run, chained from the start invert level entered on the Runs tab - useful for checking against a drainage layout drawing or handing to groundworkers on site.

Minimum fall, at a glance

Pipe diameterMin fall - foulMin fall - surface water
Approximate self-cleansing minimum fall figures, broadly following UK Building Regulations Approved Document H guidance. For foul drains, a 100mm run serving at least one WC may be laid at 1 in 80 and a 150mm run serving at least five WCs at 1 in 150 - enter the WCs each foul run serves on the Runs tab; with fewer WCs the steeper figure applies. Always confirm against the current regulations and your drainage engineer's design for the actual project. The 300mm surface water figure in particular is an extrapolation, not a published table value - published guidance recommends a full hydraulic design (catchment area and rainfall intensity) for surface water pipes at and above 225mm rather than a simple ratio.

Drainage fall calculator

Work out falls and invert levels for a drainage run. Enter a start invert level, then each run's length, fall as 1 in N, pipe size, foul or surface water, and the WCs a foul run serves. You get the fall per run, a level schedule chained run to run, a to-scale fall profile, and a flag on any run flatter than its recommended minimum.

It is free, works in your browser with no sign-up, and there is a free Windows app too.

Worked example: two-run foul drain from an invert level of 98.450m

A new foul drain starts at an invert of 98.450m. Run MH1-MH2 is 15m of 100mm pipe at 1 in 40 serving 1 WC, then MH2-MH3 is 24m of 150mm pipe at 1 in 60 serving 3 WCs.

  1. Fall = length / N. MH1-MH2: 15 / 40 = 0.375m.
  2. Invert at MH2 = 98.450 - 0.375 = 98.075m.
  3. MH2-MH3: 24 / 60 = 0.400m, so MH3 = 98.075 - 0.400 = 97.675m.
  4. Total length 39.0m, total fall 0.775m, shown rounded as 0.78m.

With 1 WC the 100mm run's minimum is 1 in 80, and with fewer than 5 WCs the 150mm run's is 1 in 60, so neither is flagged as shallow. The Levels tab lists chainages and inverts to three decimals for setting out. Treat the result as an estimate - building control or your drainage designer signs off the final falls.

Questions

What is a 1 in 40 fall in mm?

A 1 in 40 fall drops 1m for every 40m of pipe, which is 25mm per metre run (1000 / 40). Over a 10m run that is 250mm of fall.

What is a 1 in 60 and 1 in 80 fall over 1m?

Divide 1000mm by the ratio: 1 in 60 is about 16.7mm per metre and 1 in 80 is 12.5mm per metre. The bigger the second number, the flatter the pipe.

What is the minimum fall for a 100mm foul drain in the UK?

Approved Document H allows a 100mm foul drain at 1 in 80 where it serves at least one WC; the calculator applies that when you enter one or more WCs on the run, and uses 1 in 40 when it serves none. Anything flatter than the minimum is flagged as shallow.

How do you work out invert levels along a drainage run?

Start from a known invert level, work out each run's fall as length divided by the ratio, and subtract it to get the invert at the next manhole or chamber. That next invert becomes the start level for the following run, which is how the calculator builds its level schedule.

How do I convert a pipe gradient to a percentage?

Divide 100 by the ratio, so 1 in 40 is 2.5%, 1 in 60 is about 1.67% and 1 in 80 is 1.25%. To go the other way, divide 100 by the percentage to get the 1 in N figure.

Does the calculator cover surface water and soil pipe runs?

Yes - set each run to foul or surface water and choose a 100, 150, 225 or 300mm pipe, and the calculator checks it against its minimum for that size, type and, for foul runs, the WCs served (a 150mm foul run drops to 1 in 150 at five or more WCs). It works on below-ground drain runs in metres and does not size pipes for flow, so surface water drains of 225mm and above still need a proper hydraulic design.

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