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Gutter Sizing Per Smacna:

This application runs the equations laid out in the Smacna handbook. It should be used to calculate required gutter size, downspout size, and downspout spacing for a given roof area. For additional information, please see below.

Note that this application is intended as a aid only, and is provided as is. Northshore Exteriors, Inc. assumes no responsibility for problems which may arise from using this calculator and recommends manual verification of all data obtained. For further assistance, please
contact us directly.



Please Note:

  1. Make sure to select the closest city to your area to ensure the correct data is used in the calculation.
  2. Plan Width and Plan Length should be flat plan dimensions (not calculated along the slope).
  3. The slope will take input as either an integer (e.g. 3) or a ratio to 12 (e.g. 3/12).
  4. Maximum Gutter per DS refers to the maximum length of gutter that is to be served by one downspout.
  5. Tweak the Gutter Relation to Height to obtain different gutter sizes.


The calculator takes the following variables:

  1. Plan Width: The width of the rectangular roof area in feet. You may round up to the nearest foot.
  2. Plan Length: The length (alternate dimension) of the roof in feet, again rounded up.
  3. Slope: The slope of the roof. (e.g. 3/12 or 3)
  4. Total Gutter Length: The length of all gutter serving this roof in feet (rounded up).
  5. Maximum Gutter Per DS: The maximum length of gutter served by one downspout in feet (rounded).
  6. Storm Requirement: The type of storm to design for.
  7. Gutter Width Relation To Height: The relation of width to height. (e.g. Width = .875 * Height)
  8. Downspout Type: The type of downspout to be used.


The calculator outputs the following information:

  1. Rainfall Intensity (10yr): The rainfall intensity lasting 5 minutes for storms occurring in your area every 10 years.
  2. Rainfall Intensity (100yr): The rainfall intensity lasting 5 minutes for storms occurring in your area every 100 years.
  3. Drainable Area (10yr): The amount of roof area that can be drained by one square inch of downspout in your area for storms which occur once every 10 years.
  4. Drainable Area (100yr): The amount of roof area that can be drained by one square inch of downspout in your area for storms which occur once every 100 years.
  5. Year Setting: The requirements you have set for storm frequency.
  6. Plan Area: The plan area of your roof.
  7. Gutter Length: The total gutter length you entered.
  8. Max Gutter Served By Each DS: The value that you entered for Maximum Gutter Per DS.
  9. Design Area: The actual gutter design area of your roof as calculated from Plan Area and Roof Slope.
  10. Minimum Number of DS: The absolute minimum number of downspouts your gutter can have based on your input.
  11. Max Roof Area Served By Each DS: The maximum area of roof that each downspout can handle.
  12. Min Gutter Width: The minimum width for your gutter based on your ratio setting.
  13. Min Gutter Height: The minimum height for your gutter based on your ratio setting.
  14. Min DS Size: The minimum downspout size based on your downspout setting.


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