Gutter and downspout flow calculator.

Formula, source, and limits

Method v1.0, reviewed September 30, 2026: flow Q (gpm) = 0.0104 × rainfall intensity (in/hr) × area (sq ft). This is the rational method with a runoff coefficient of 1. The constant is unit conversion: 1 in of rain on 1 sq ft is 144 cu in, and 144 ÷ 231 cu in per U.S. gallon = 0.623 gal; divided by 60 min gives 0.01039 gpm per (in/hr × sq ft), rounded to 0.0104 for display while the calculation uses the exact value. Sloped area = plan footprint × sqrt(1 + (rise/12)²).

Area basis: this tool does not assert a single convention. Horizontal plan footprint is the default and sloped surface area is offered as the conservative alternative; your local plumbing or building code decides which applies. The SMACNA page below describes a Design Area Wizard and vertical-wall area handling per the International Plumbing Code, which are not modeled here.

Rainfall intensity: paste the value for your location, duration and return period from the NOAA Atlas 14 Precipitation Frequency Data Server (choose intensity in inches per hour), accessed September 30, 2026. Capacity: enter the downspout or gutter rating from the manufacturer or your code. No capacity table is reproduced here. For full sizing see the SMACNA Downspout & Gutter Sizing Calculator, which applies the Architectural Sheet Metal Manual methods, accessed September 30, 2026.

Boundary: flow and load only. Gutter shape, slope, outlets, debris, vertical wall runoff and code rainfall rates are not modeled. This is not engineering or code advice.

Frequently asked questions