Estimate the required total net opening area for non-engineered flood openings, the enclosure area supported, or the count of identical openings using the 1 in² per 1 ft² sizing rule.

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Non-engineered openings only. Use net clear opening area. Each enclosure needs at least two exterior openings; placement and local approval still apply.

Flood Vent Size Formula

The following example problem outlines the steps and information needed to calculate the Flood Vent Size.

FVS = A × ((1 in²) / (1 ft²))

Variables:

  • FVS is the required total net open area of flood vents (square inches)
  • A is the enclosed area requiring flood openings (ft2) 

For non-engineered flood openings, use the prescriptive minimum of 1 square inch of net open area per 1 square foot of enclosed area. Net area excludes solid grilles, louvers and faceplates; gross vent dimensions are not sufficient. Each qualifying enclosure needs at least two exterior openings, with placement, automatic water entry and exit, and local approval also required. Engineered openings instead use certified model coverage. Under the non-engineered rule, required net in² numerically equals enclosed ft²; this is a sizing rule, not a geometric unit conversion.

How to Calculate Flood Vent Size?

The following steps outline how to calculate the Flood Vent Size.

  1. First, determine the enclosed area (ft2) requiring flood openings. 
  2. Next, gather the formula from above: FVS = A × (1 in2/ft2).
  3. Finally, calculate the Flood Vent Size.
  4. 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.

enclosed area (ft2) = 900

FVS = 900 ft2 × (1 in2/ft2) = 900 in2 of total net opening area.