Gutter Size Calculator
Gutter Size is evaluated from Roof Width, Roof Section Length and Roof Pitch. The calculation reports Effective Drainage Area, Peak Flow Rate and Recommended Gutter Size.
Results
About the Gutter Size Calculator
The calculator uses a multi formula configuration. Each reported value is read as a direct evaluation of the stored rules with the declared field formats and units.
Formula basis:
Effective drainage area = (roof half-width) x roof length x pitch adjustment factor
Pitch adjustment: steeper roofs concentrate flow faster; factor = 1 + 0.5 x (pitch - 3)/12 for pitches >3/12
Peak flow rate (gpm) = drainage area (sq ft) x rainfall intensity (in/hr) / 96.23
(Conversion: 1 in/hr over 1 sq ft = 0.01039 gal/min)
Gutter size: if flow > 50 gpm → 6-inch; otherwise 5-inch
Downspout spacing: maximum 35 - 40 LF between downspouts (one downspout per 35 LF rule of thumb)
Interpret the outputs in the order shown by the result fields. Optional inputs affect only the outputs that depend on those variables.
Formula & How It Works
The calculation applies the following relations exactly as recorded in the metadata: Effective drainage area = (roof half-width) x roof length x pitch adjustment factor Pitch adjustment: steeper roofs concentrate flow faster; factor = 1 + 0.5 x (pitch - 3)/12 for pitches >3/12 Peak flow rate (gpm) = drainage area (sq ft) x rainfall intensity (in/hr) / 96.23 (Conversion: 1 in/hr over 1 sq ft = 0.01039 gal/min) Gutter size: if flow > 50 gpm → 6-inch; otherwise 5-inch Downspout spacing: maximum 35 - 40 LF between downspouts (one downspout per 35 LF rule of thumb) Each output field is produced by substituting the supplied inputs into the relevant relation and then applying the declared rounding or text format.
Worked Examples
Example 1: Ranch house: 28 ft wide × 40 ft long, 5/12 pitch, 4 in/hr rainfall
Inputs
With Roof Width = 28, Roof Section Length = 40, Roof Pitch = 5 and Max Rainfall Intensity = 4 as the stated inputs, the result is Effective Drainage Area = 607 ft^2, Peak Flow Rate = 25 gpm and Recommended Gutter Size = 5 in. Each value corresponds to the declared output fields.
Example 2: Large colonial: 40 ft wide × 60 ft long, 8/12 pitch, 5 in/hr (Gulf South)
Inputs
With Roof Width = 40, Roof Section Length = 60, Roof Pitch = 8 and Max Rainfall Intensity = 5 as the stated inputs, the result is Effective Drainage Area = 1,450 ft^2, Peak Flow Rate = 75 gpm and Recommended Gutter Size = 6 in. Each value corresponds to the declared output fields.
Example 3: Shed: 12 ft × 16 ft, 4/12 pitch, 2 in/hr
Inputs
With Roof Width = 12, Roof Section Length = 16, Roof Pitch = 4 and Max Rainfall Intensity = 2 as the stated inputs, the result is Effective Drainage Area = 100 ft^2, Peak Flow Rate = 2 gpm and Recommended Gutter Size = 5 in. Each value corresponds to the declared output fields.
Example 4: Townhouse end unit: 18 ft wide × 30 ft long, 6/12 pitch, 3.5 in/hr
Inputs
With Roof Width = 18, Roof Section Length = 30, Roof Pitch = 6 and Max Rainfall Intensity = 3.5 as the stated inputs, the result is Effective Drainage Area = 304 ft^2, Peak Flow Rate = 11 gpm and Recommended Gutter Size = 5 in. Each value corresponds to the declared output fields.
Common Use Cases
- Determine gutter size for a steep roof in a high-rainfall area
- Calculate linear feet of gutter needed for a house
- Determine number of downspouts for a gutter run