This Calculator will help you to determine the proper amount of ventilation based on the VA/FHA Guidelines. To use simply enter the dimension or square footage of your attic and click 'Update.' The calculations are done for you. The products listed are GAF Master Flow® products and the quantity numbers are rounded up or down for your convenience.
Please note that the 1/300 or the 1/150 rule are general rules and do not apply to all situations. Local building codes, when stricter, take precedence. Always consult a design professional for cathedral ceilings, insulated decks, etc.
Using the FHA 1/300 rule
384.00 net free vent needed at ridge
384.00 net free vent needed at soffit
Using the FHA 1/150 rule
(vents placed at or near the ridge only)
1536.00 net free vent area needed