1 Your measurements

Type feet and inches the way you'd write them, like 12' 6" or 30 3/8. Decimals work too.

ft
Measure the attic floor, which is usually the house footprint under the roof (not counting overhangs).
ft
Use 1/150 unless you meet the 1/300 conditions explained below the calculator.

2 Vents you need

Net free vent area
1,152 sq in
8 sq ft, at 1/150 of 1,200 sq ft
Ridge vent
32 ft
exhaust: 576 sq in
Continuous soffit vent
64 ft
intake: 576 sq in
Attic floor area1,200 sq ft
Code ratio1/150 (2021 IRC R806.2 minimum)
Total net free area needed1,152 sq in, half exhaust and half intake
Exhaust (upper)576 sq in ÷ 18 sq in per ft = 32 ft of ridge vent (576 sq in installed)
Intake (lower)at least 576 sq in ÷ 9 sq in per ft = 64 ft of continuous soffit vent (576 sq in installed, never less than the exhaust)

Net free area differs between products, so check the label on the vents you buy and divide by its number. Use one type of exhaust vent per attic: Air Vent warns that mixing a ridge vent with gable vents, louvers or a fan short-circuits the airflow, and DOE says the same about gable vents.

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