Use our CFM calculators and charts to determine how many vents you need for your project.

AV Diameters

Enter the length and width (in feet) or total square footage of your building in the fields below to determine how many vents are required based on the 1/150 or 1/300 Rule. This calculator is for the EXHAUST HALF ONLY. If an intake ventilator is needed, just double the amount of vents. For example, if 2 AV-14 are needed and inlet vents are needed, then an additional 2 PV-14 vents would be required

PV Diameters

UV Diameters

AF Diameters


Step 1: Calculate the square footage of the attic (feet)

Step 2: Enter the height at the attic's highest point (feet)

Step 3: Choose your roof type

Total cubic footage of your attic:

Step 4: Choose air change rate per hour

Total cubic feet of air to be exhausted per hour:

Step 5: Pick the fan to be used


Total number of fans required:

RBV Diameters