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Storm Water Drainage Calculator

Work out storm water drainage instantly with clear inputs, formula shown and shareable results.

About Storm Water Drainage Calculator

A pipe running full has hydraulic radius D/4, so Manning gives velocity directly. A 300 mm concrete pipe at 1 in 200 carries 0.062 m3/s at 0.88 m/s - just under the 0.9 m/s self-cleansing threshold, so a steeper gradient would be specified to keep solids moving.

Formulas

Full bore hydraulic radius

R = D / 4 for a circular pipe running full

Capacity

Q = (pi D^2 / 4) x (1/n) x (D/4)^(2/3) x S^(1/2)

Worked example

With the default values below, Storm Water Drainage Calculator returns Full Bore Capacity of 0.0684. Change any input above and every figure here is recalculated in your browser.

Inputs used

Pipe diameterminimum 1 mm
300 mm
Pipe gradientminimum 0
0.005
Manning roughness nminimum 0
0.013

Results

Full Bore Capacity
0.0684 m3/s
Same Capacity In Litres Per Second
68.4
Full Bore Velocity
0.967 m/s
Self-Cleansing Check
Above the 0.9 m/s self-cleansing velocity

Step by step

  1. 1Values used
  2. 2Full bore hydraulic radius
  3. 3Capacity
  4. 4Full Bore Capacity= 0.0684 m3/s
  5. 5Same Capacity In Litres Per Second= 68.4
  6. 6Full Bore Velocity= 0.967 m/s
  7. 7Self-Cleansing Check= Above the 0.9 m/s self-cleansing velocity

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 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

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

Frequently Asked Questions

Why does a part-full pipe carry more than a full one?

Because at about 94% depth the hydraulic radius peaks. Capacity there slightly exceeds the full-bore figure.

What is the minimum gradient for a 150 mm drain?

Commonly 1 in 80 for foul drainage to maintain self-cleansing velocity at low flows.

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