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Drainage Fall Calculator

Work out a drain pipe's fall, gradient, and end depth, with a self-cleansing range check.

  • drainage
  • drain
  • fall
  • gradient
  • slope
  • pipe
  • invert
  • sewer
  • plumbing

About Drainage Fall Calculator

A buried drain only works if it falls at the right rate. Too little and waste water creeps along, drops its solids, and silts up; too much and — for a foul drain — the liquid races ahead and strands the solids behind it. The sweet spot is a gentle, consistent gradient, and this calculator turns a pipe length and a target fall into the numbers you actually set out on site: the total drop, the gradient as a ratio and as millimetres per metre, and the depth the pipe reaches at its far end.

It expresses the same fall three ways because everyone quotes it differently — a percentage, a 1-in-n ratio, or a fall per metre — and checks your gradient against the typical self-cleansing band so you know at a glance whether it's too shallow, about right, or too steep before you start digging the trench.

How to use

Enter the pipe length and the fall you're aiming for as a percentage, or tap one of the common drainage presets (1:40, 1:60, 1:80, 1:100). If you know the invert depth — the depth to the bottom of the pipe — at the start, add it and the calculator works out the depth at the far end so you can confirm the run still clears its connection or outfall.

The headline gives the total vertical drop over the run, with the gradient as a ratio, a fall per metre, and a percentage beneath it. The advisory banner flags whether the gradient is in the usual self-cleansing range. Gradient requirements depend on the pipe diameter and whether it's carrying foul or surface water, so treat the band as guidance and confirm the figure against your local plumbing or drainage code.

Frequently asked questions

  • What fall should a drain pipe have?

    A common rule for a 100 mm foul drain is a gradient between 1:80 and 1:40 — that is, 1.25% to 2.5%, or 12.5 to 25 mm of drop per metre. Larger 150 mm pipes can run flatter, around 1:150, because they carry more flow. Surface-water drains are more forgiving. These are typical figures: the exact requirement depends on pipe size and your local code, so always confirm before building.

  • How do I calculate the fall of a drain?

    Multiply the pipe length by the gradient. At a 1:80 gradient (1.25%), a 20 m run falls 20 ÷ 80 = 0.25 m, or 250 mm. The calculator does this and also shows the fall as millimetres per metre and as a percentage, so you can transfer whichever figure your level, laser, or string line is set up to use.

  • What is the invert level of a drain?

    The invert is the inside bottom of the pipe — the lowest point of its bore — and it's what drainage levels are actually set to, not the top or centre of the pipe. If you enter the start invert depth, this tool adds the total fall to give the invert depth at the far end, which you can check against the depth your connection, manhole, or outfall sits at to confirm the run works.

  • What happens if a drain is too shallow or too steep?

    Too shallow and the flow is too slow to be self-cleansing: solids settle out, build up, and eventually block the pipe. Too steep — for a foul drain — and the water can run away faster than the solids, leaving them stranded, which causes the same blockage from the other direction. The ideal gradient keeps the flow fast enough to scour the pipe but slow enough to carry everything together.

  • Does this work for surface water as well as foul drains?

    Yes, for the geometry — the fall, gradient, and end depth are the same maths whatever the pipe carries. The self-cleansing band the tool flags is aimed at foul drains, which are the fussier case. Surface-water and storm drains tolerate steeper gradients without trouble since they don't carry solids the same way, so a "steep" flag matters less for them.

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