Rain Gutter Calculator
Size gutters and downspouts from roof area and rainfall intensity.
About Rain Gutter Calculator
Rainfall lands on the horizontal projection, so roof pitch does not increase the volume collected. Downspout count usually governs the design because a single downspout carries far less than a long gutter run delivers. Undersized drainage overflows at the gutter rather than backing up visibly, which is how water ends up against the foundation.
Formula
Rain Gutter
Flow in L/s = roof plan area × rainfall intensity ÷ 3,600
- plan area
- Horizontal projection of the roof, not the sloped surface area
- design intensity
- Short-duration rainfall rate for the local design storm
Worked example
With the default values below, Rain Gutter Calculator returns Design Flow of 2.500. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Roof plan area drainedminimum 0 m2
- 120 m2
- Design rainfall intensityminimum 1 mm/h
- 75 mm/h
- Gutter capacity per runminimum 0.01 L/s
- 1.2 L/s
- Downspout capacityminimum 0.01 L/s
- 2.5 L/s
Results
- Design Flow
- 2.500 L/s
- Downspouts Required
- 1
- Gutter Runs Required
- 3
- Design Flow
- 9,000 L/h
- Roof Area per Downspout
- 120.00 m2
Step by step
- 1Values used
- 2Rain Gutter
- 3Design Flow= 2.500 L/s
- 4Downspouts Required= 1
- 5Gutter Runs Required= 3
- 6Design Flow= 9,000 L/h
- 7Roof Area per Downspout= 120.00 m2
Assumptions and limits
- The result is derived only from the 4 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 Rain Gutter calculated?
Flow in L/s = roof plan area × rainfall intensity ÷ 3,600 Rainfall lands on the horizontal projection, so roof pitch does not increase the volume collected. Downspout count usually governs the design because a single downspout carries far less than a long gutter run delivers.
Why does Rain Gutter matter?
Undersized drainage overflows at the gutter rather than backing up visibly, which is how water ends up against the foundation.
What values do I need to enter?
This calculator takes 4 inputs: Roof plan area drained, Design rainfall intensity, Gutter capacity per run, Downspout capacity. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.