Free roofing tool

Roof ventilation calculator

Find how much attic ventilation your roof needs, using the 1/300 code rule.

Proper attic ventilation protects your roof and cuts cooling costs. Enter your attic floor area to see the net free vent area (NFA) you need, split evenly between intake and exhaust.

Attic floor area (sq ft) The footprint of your attic — usually close to your home’s floor area. Ventilation rule

Net free vent area needed

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A guide only — local code and roof design set the final requirement.

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How attic ventilation is figured

Code uses the 1/300 rule: you need 1 sq ft of net free vent area (NFA) for every 300 sq ft of attic floor when intake and exhaust are balanced and you have a vapor barrier — otherwise 1/150. Split the NFA roughly 50% intake (soffit) and 50% exhaust (ridge). Each vent product lists its NFA in square inches.

Cost ranges reflect 2026 U.S. installed prices (BLS producer/labor data, Remodeling Cost vs. Value, and manufacturer pricing). Your real price depends on your roof, region, and pro — see the full cost guide.

FAQ

Roof Ventilation Calculator questions

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How much attic ventilation do I need?

Under the 1/300 rule, a 1,500 sq ft attic needs about 5 sq ft of net free vent area (720 sq in) — roughly half intake at the soffits and half exhaust at the ridge.

What is net free area (NFA)?

NFA is the actual open area a vent provides for airflow, after screens and louvers. It is printed on every vent’s packaging in square inches, so you can add up what you have.

Is more roof ventilation always better?

No — balance matters more than sheer volume. Too much exhaust without matching intake can pull air from the living space. Aim for a 50/50 intake-to-exhaust balance.

What happens with poor attic ventilation?

Heat and moisture build up, shortening shingle life, warping decking, and feeding mold and ice dams. Good ventilation is one of the cheapest ways to protect a roof.

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