Roof Vent Calculator

Proper attic ventilation prevents heat buildup, moisture damage, and premature roof aging — but calculating how much venting you actually need requires matching your attic size to building code standards. Enter your attic length, attic width, building code standard (1/300 general or 1/150 high-moisture), primary vent type, and roofing material into the Roof Vent Calculator to find the total net free vent area required. Secondary outputs include intake vents needed, exhaust vents needed, and ridge vent length required.

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Optional: Override calculated area

Results

Total Net Free Vent Area Required

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Intake Vents Needed (50%)

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Exhaust Vents Needed (50%)

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Ridge Vent Length Required

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Frequently Asked Questions

What is the 1/300 attic ventilation rule?

The 1/300 rule requires 1 square foot of net free vent area for every 300 square feet of attic floor space. This is the standard building code requirement for most residential applications.

When should I use the 1/150 ventilation standard instead?

The 1/150 rule (double the ventilation) is required in high moisture areas, by some local building codes, or when vapor barriers are not properly installed. Always check your local building codes.

How should intake and exhaust vents be balanced?

Proper attic ventilation requires 50% of the net free area at the intake (soffit/eave vents) and 50% at the exhaust (ridge or roof vents) to create effective airflow.

What is Net Free Vent Area (NFVA)?

NFVA is the actual open area available for airflow after accounting for screens, louvers, and other obstructions. It's always less than the gross vent opening size.

Can I mix different types of exhaust vents?

While possible, it's generally not recommended to mix different exhaust vent types as they can interfere with each other's performance. Stick to one primary exhaust system.

How do I calculate ridge vent requirements?

Most ridge vents provide about 18 square inches of NFVA per linear foot. Divide your required exhaust area by 18 to get the linear feet of ridge vent needed.

Do power vents require different calculations?

Power vents move more air than passive vents, but they still need adequate intake ventilation to function properly. Follow manufacturer specifications for CFM ratings and intake requirements.