The below commentary and instruction may be used as a pre-Revit calculation or as a post Revit conformation.

Attic Ventilation Calculation:

Per the 2020 Building Code section 1203.2 and Exception 2, we need to provide ventilation that is a minimum of 1/300 of the total area attic area or attic compartment.  To meet the exception, we also need to make sure the amount of ventilation is balanced to provide 40-50% of the total ventilation from the upper portion of the roof and the remaining from the lower portion of the roof.  Based on the attic area, the spreadsheet will automatically calculate the required ventilation area and then by adjusting the size or number of vents you can determine what ventilation is needed.

First input the area of the attic to be evaluated.  This will produce a range of required upper ventilation that is between 40-50% of the total ventilation.  Adjust the amount of ridge vent and/or off-ridge vents to get an area that is within the 40-50% range.  The remaining ventilation is required to be provided by the lower ventilation.  Adjust the amount of soffit vent to be provided to meet or exceed the remaining required area.  The section at the bottom shows the proof of the calculation that the percentage of the upper ventilation is within the 40-50% range and the total ventilation meets or exceed the total required.