Enter the rain intensity and the contributing drainage (catchment) area (such as the roof plan area draining to the gutter) into the calculator to estimate the runoff flow rate delivered to a gutter during a rain event. Note: a gutter system’s actual conveyance capacity also depends on gutter profile, slope, length, outlet/downspout capacity, and debris.

Gutter Flow Rate Calculator

Basic Rational Method Gutter Area Helper Gutter Size & Downspouts

Enter any 2 values to calculate the missing variable (assumes 100% runoff from the contributing area).

Related Calculators

Gutter Flow Rate Formula

The following formula is used to estimate the runoff flow rate delivered to a gutter from rainfall on a contributing (catchment) area.

FR = (RI / 60) * (GA / 231)

Variables:

  • FR is the flow rate (gallons per minute)
  • RI is the rain intensity (inches per hour)
  • GA is the contributing drainage (catchment) area (square inches)

To calculate the flow rate, multiply the rain intensity (in/hr) by the contributing drainage area (in²) to get a volumetric rate in cubic inches per hour, then divide by 231 cubic inches per US gallon and by 60 minutes per hour to convert to gallons per minute (GPM). This estimates runoff inflow; a gutter’s actual conveyance capacity can be lower depending on slope, outlets/downspouts, and obstructions.

What is Gutter Flow Rate?

Gutter flow rate is the volumetric rate of water delivered to (and carried by) a gutter system, typically measured in gallons per minute (GPM). For estimating the runoff inflow from a roof, it is primarily determined by the rain intensity and the contributing roof drainage (catchment) area. The conveyance capacity of a specific gutter and downspout system additionally depends on gutter profile and dimensions, slope, run length, outlet/downspout sizing and layout, and debris/icing.

How to Calculate Gutter Flow Rate?

The following steps outline how to calculate the Gutter Flow Rate.

  1. First, determine the rain intensity (RI) in inches per hour.
  2. Next, determine the contributing drainage (catchment) area (GA). If your area is in square feet, square meters, etc., convert it to square inches for use in the formula.
  3. Next, gather the formula from above = FR = (RI / 60) * (GA / 231).
  4. Finally, calculate the Gutter Flow Rate (FR) in gallons per minute.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

Rain intensity (RI) = 1 inch per hour

Drainage (catchment) area (GA) = 100 square feet (= 14,400 square inches), which gives FR ≈ 1.039 gallons per minute.