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Gutter Size Calculator

Work out gutter size instantly with clear inputs, formula shown and shareable results.

About Gutter Size Calculator

One millimetre of rain on one square metre is one litre, so runoff in litres per hour is plan area times intensity, and dividing by 3600 gives litres per second. The gutter is then selected on the flow reaching each outlet, since a gutter with outlets at both ends carries only half the total at any point.

Formulas

Design flow

L/s = roof plan area x rainfall intensity mm/h / 3600

Per outlet

Flow per outlet = total flow / number of outlets

Worked example

With the default values below, Gutter Size Calculator returns Design flow to the gutter of 2.083. 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 outletsminimum 1
2

Results

Design flow to the gutter
2.083 L/s
Flow per outlet
1.042 L/s
Gutter size suggested
100 mm half round
Runoff volume
7,500 L/h

Step by step

  1. 1Values used
  2. 2Design flow
  3. 3Per outlet
  4. 4Design flow to the gutter= 2.083 L/s
  5. 5Flow per outlet= 1.042 L/s
  6. 6Gutter size suggested= 100 mm half round
  7. 7Runoff volume= 7,500 L/h

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 Gutter Size figure worked out?

L/s = roof plan area x rainfall intensity mm/h / 3600. One millimetre of rain on one square metre is one litre, so runoff in litres per hour is plan area times intensity, and dividing by 3600 gives litres per second.

Should the sloping roof area be used?

No. Use the plan projected area, because rain falls vertically. A steeply pitched roof collects the same rainfall as its footprint, though wind-driven rain justifies a small increase.

What values do I need to enter?

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

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Gutter Size Calculator – Calculate Design flow to the gutter