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Downpipe Sizing Calculator

Work out downpipe sizing instantly with clear inputs, formula shown and shareable results.

About Downpipe Sizing Calculator

Rainwater downpipes run part full with an annular film, and a working design velocity of about 1.5 m/s gives the required bore: area in square millimetres is roughly 667 times the flow in litres per second. The result is rounded up to the next standard pipe size.

Formulas

Flow per pipe

L/s = area x intensity / 3600 / number of downpipes

Bore

Area mm² = flow L/s x 667; diameter = sqrt(4A / pi)

Worked example

With the default values below, Downpipe Sizing Calculator returns Downpipe diameter required of 29.7. Change any input above and every figure here is recalculated in your browser.

Inputs used

Roof plan area drainedminimum 0.1 m²
100 m²
Design rainfall intensityminimum 1 mm/h
75 mm/h
Number of downpipesminimum 1
2

Results

Downpipe diameter required
29.7 mm
Flow per downpipe
1.042 L/s
Total roof runoff
2.083 L/s
Nominal pipe size
50 mm nominal

Step by step

  1. 1Values used
  2. 2Flow per pipe
  3. 3Bore
  4. 4Downpipe diameter required= 29.7 mm
  5. 5Flow per downpipe= 1.042 L/s
  6. 6Total roof runoff= 2.083 L/s
  7. 7Nominal pipe size= 50 mm nominal

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 3 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

How is the Downpipe Sizing figure worked out?

L/s = area x intensity / 3600 / number of downpipes. Rainwater downpipes run part full with an annular film, and a working design velocity of about 1.5 m/s gives the required bore: area in square millimetres is roughly 667 times the flow in litres per second.

Why not use one large downpipe?

Two smaller pipes give redundancy if one blocks with leaves, and they halve the flow each gutter run has to carry, which often allows a smaller gutter.

What values do I need to enter?

This calculator takes 3 inputs: Roof plan area drained, Design rainfall intensity, Number of downpipes. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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