Gutter & Downspout Sizing Calculator

Inputs

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Plan roof area draining to this gutter/downspout set.
ft²
Use the local design rainfall intensity for the required duration and return period.
in/h
Roof runoff coefficient. Use 1.0 for a first-pass roof screen.
Allowance for steep roof, wind-driven rain, valley concentration or conservative design criteria.
Extra factor applied to roof runoff for debris, leaf guards, partial blockage and uncertainty.
%
Number of gutter runs sharing this roof area.
Approximate inside flow width of the gutter.
in
Usable water depth before overflow, not necessarily the nominal face size.
in
Gutter fall divided by horizontal run, times 100.
%
Roughness coefficient for a clean metal or vinyl gutter screen. Use manufacturer data when available.
Number of downspouts serving the entered roof area.
Inside or effective width of one rectangular downspout opening.
in
Inside or effective height of one rectangular downspout opening.
in
Effective open-area allowance for outlet, strainer, elbow entrance and shape effects.
%
Area-velocity screen used to convert downspout open area into first-pass capacity.
ft/s

Results

Default result
Edit inputs
Design roof runoff(Qd)
82.29gpm
Pass

Raw runoff before allowances: 259.6 L/min.

Also computed

Flow per gutter run(Qg)Pass41.14gpm

2 gutter run(s) sharing the design runoff.

Gutter capacity per run(Qg cap)Pass100.4gpm

Rectangular open-channel Manning capacity at the entered depth and slope.

Gutter utilization(Ug)Pass40.96%

Required gutter depth(y req)Pass1.792in

Depth needed at the entered gutter width, slope and Manning n.

Downspout capacity(Qdsp)Caution101gpm

Area-velocity screen for the entered downspout openings.

Downspout utilization(Ud)Caution81.48%

Gutter sizing preview: gutter 41 percent used, downspouts 81 percent usedRoof drainage previewDesign runoff split across gutter runs and downspoutsdesign runoff311 L/minper gutter run156 L/minGutter41%DownspoutsScreen only. Local rainfall, code, roof shape, gutter profile, outlets, debris and manufacturer data control final sizing.
Method notes 4 notes
  • Roof runoff screen: Q = i * A * C * F, with user-entered rainfall intensity, runoff coefficient, roof factor and safety factor.
  • Gutter capacity uses a rectangular open-channel Manning approximation. Real K-style, half-round and custom profiles should be checked against manufacturer capacity data where available.
  • Downspout capacity uses effective open area times a target velocity. Elbows, strainers, leaf guards, outlet transitions, leader pipe, splash, ice, debris and local code can control final sizing.
  • Final roof drainage still needs local rainfall criteria, roof geometry, overflow/scupper routing, fascia/slope constraints, discharge location and jurisdiction requirements.

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