Gutter Capacity Calculator
Estimate a rectangular gutter flow capacity with Manning’s equation and compare it with roof runoff from catchment area and rainfall intensity.
Default project preview
69.7 gpm
Base roof runoff
37.4 gpm
Design runoff
44.9 gpm
Capacity utilization
64%
Flow margin
24.8 gpm
Calculator
Build the material order
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Enter the project details and calculate a rounded material estimate.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Manning flow Q = (1/n) × area × hydraulic radius^(2/3) × slope^(1/2). Roof runoff = roof area × rainfall intensity, increased by the entered safety margin.
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BuildMeter Guides
Understand the measurements and assumptions
Use these visible reference guides before changing project inputs or planning allowances.
FAQ
Gutter Capacity Calculator questions
Why use a planning water depth smaller than the full gutter depth?+
Freeboard helps reduce overtopping from waves, debris, outlet restrictions and imperfect slope.
What roughness value should be entered?+
Use a value appropriate to the gutter material and condition. The default is a smooth-channel planning assumption.
Does this size downspouts?+
No. Downspout outlets and spacing must also carry the design flow and can control the overall system capacity.