Enter the rain intensity and the area of the gutter into the calculator to determine the gutter flow rate. This calculator helps in estimating the amount of water that can flow through a gutter system during a rain event.

Gutter Flow Rate Formula

The following formula is used to calculate the gutter flow rate.

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


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

To calculate the gutter flow rate, convert the rain intensity from inches per hour to gallons per minute, then multiply by the gutter area in square inches divided by 231 (since 1 gallon is approximately 231 cubic inches).

What is Gutter Flow Rate?

Gutter flow rate is the volume of water that can flow through a gutter system per unit of time, typically measured in gallons per minute (GPM). It is an important consideration in the design of gutter and downspout systems for buildings, as it affects the system’s ability to handle rainfall without overflowing. The flow rate is determined by the rain intensity and the cross-sectional area of the gutter.

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 gutter area (GA) in square inches.
  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

Gutter area (GA) = 100 square inches