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Drain Pipe Slope

Free Drain Pipe Slope calculator with clear step-by-step results.

About Drain Pipe Slope

Converts a drainage slope into the total fall you have to build in, expressed three ways because plumbing codes use all three - percentage, millimetres per metre and one-in-x. The check compares your slope against the usual minimum for that diameter, since larger pipes need less fall to stay self-cleansing.

Formulas

Total fall

Fall mm = run length m x slope x 1000

Gradient as a ratio

1 in x, where x = 1 / slope as a decimal

Worked example

With the default values below, Drain Pipe Slope returns Total fall over the run of 240.0. Change any input above and every figure here is recalculated in your browser.

Inputs used

Horizontal run lengthminimum 0.1 m
12 m
Pipe internal diameteraccepts 20–600 mm
110 mm
Slopeaccepts 0.1–20 %
2 %

Results

Total fall over the run
240.0 mm
Fall per metre
20.0 mm/m
Gradient expressed as 1 in
50.0
Gradient check
Meets the usual minimum gradient

Step by step

  1. 1Values used
  2. 2Total fall
  3. 3Gradient as a ratio
  4. 4Total fall over the run= 240.0 mm
  5. 5Fall per metre= 20.0 mm/m
  6. 6Gradient expressed as 1 in= 50.0
  7. 7Gradient check= Meets the usual minimum gradient

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Drainage gradients, ventilation and pipe sizing are governed by plumbing codes that vary by jurisdiction. Work should be designed and inspected to the applicable standard.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Drainage gradients, ventilation and pipe sizing are governed by plumbing codes that vary by jurisdiction. Work should be designed and inspected to the applicable standard.

Frequently Asked Questions

Can a drain be too steep?

Yes. Above roughly 5% the water can outrun the solids, leaving them behind to build up. Steep drops are better handled with a vertical stack than a long steep run.

Why do bigger pipes need less slope?

Self-cleansing depends on flow velocity, and a larger pipe reaches the same velocity at a shallower gradient for a given flow. Oversizing a lightly loaded drain is a common cause of blockages.

Related Calculators

Drain Pipe Slope – Calculate Total fall over the run