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
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Enter the project details and calculate a rounded material estimate.
Input and output guide
What each value means
The labels and units below are part of this calculator’s visible methodology. Replace every default with measurements or product data from the actual project.
Gutter water-surface width
Enter this project assumption in in. The current imperial default is 5 in.
Planning water depth
Enter this project assumption in in. The current imperial default is 2.5 in.
Gutter slope
Enter this project assumption in %. The current imperial default is 0.5 %.
Manning roughness coefficient
Enter this project assumption in n. The current imperial default is 0.013 n.
Roof catchment area
Enter this project assumption in ft². The current imperial default is 1200 ft².
Planning rainfall intensity
Enter this project assumption in in/hr. The current imperial default is 3 in/hr.
Runoff safety margin
Enter this project assumption in %. The current imperial default is 20 %. Minimum: 0. Maximum: 100.
How to read the outputs
Base roof runoff
roof area × rainfall intensity. Default preview: 37.4 gpm.
Design runoff
20% margin included. Default preview: 44.9 gpm.
Capacity utilization
within estimated channel capacity. Default preview: 64%.
Flow margin
remaining capacity. Default preview: 24.8 gpm.
Flow cross-section
width × planning water depth. Default preview: 12.5 in².
Hydraulic radius
flow area ÷ wetted perimeter. Default preview: 1.25 in.
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.
Related tools
Continue planning the project
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.