Ventilation Calculator

Calculating Ventilation Requirements:

Most industry authorities and Local Building Codes recommend a minimum of 1/300th of horizontal roof area be provided in ventilation.

Use the Duraflo Ventilation Calculator to determine your home’s ventilation requirements. The calculator will determine the number of square inches of NET FREE AREA required for adequate attic ventilation based on the 1/300th general requirement.

Here’s how to determine your requirements:

Step 1

Determine the length and width of the space you desire to ventilate.

Be sure to factor the amount of soffit overhang into your calculation.

Step 2

Choose a roof ventilation product. Determine the products square inches of net free area and enter it in the below box.

Step 3

Based on your calculations, and product choice, you require the below

number of ventilation units for your project:

Now you can select the appropriate Duraflo ventilators by referring to the NET SQUARE INCHES ratings for different models within our catalogue.

For proper ventilation balance:

  • 50% of your total NET FREE AREA should be vented from the soffit area and
  • 50% should be vented from the roof area.


The horizontal roof area of your home (including overhang) is 36 feet by 28 feet. In this example the Duraflo NFA Calculator will provide your NET FREE AREA as 483.

You require 483 square inches of ventilation, ideally:

  • 241.5 square inches spread out in the soffit areas and
  • 241.5 square inches located in the roof area.

Note: In this example, rounding off your “roof area” requirements to 250 sq. in. you would need 5 duraflo 6050-50 sq. in. roof vents.

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Contact Information

A&G Roofing

4277 Carlyon Line,

Severn, ON,

L3V 0W8

Hours of Operation
Mon – Fri: 8:00am – 4:00 pm