Ridge Vent Length Calculator
Calculate the ridge vent length an attic needs for balanced ventilation from the attic area and the required net free area.
How it works
total NFA (sq in) = attic area ÷ ratio × 144 · ridge share = 50% of total · ridge length = ridge NFA ÷ vent NFA per foot
Attic ventilation codes require a net free area of 1 sq ft of vent per 150 sq ft of attic, or 1 per 300 when a vapor barrier balances intake and exhaust. Convert that to square inches and split it evenly — half as high exhaust at the ridge, half as low intake at the soffits — so air moves. Divide the ridge half by the vent product’s rated net free area per foot (often 12–18 sq in/ft) to get the length to install, and provide at least as much soffit intake.
Worked example
A 1,500 sq ft attic at 1:300 needs 5 sq ft = 720 sq in of net free area. Half, 360 sq in, is ridge exhaust; at 18 sq in per foot that is 20 ft of ridge vent, balanced by 360 sq in of soffit intake.
Frequently asked questions
What is net free area?
It is the actual open area for airflow after the vent’s screen, baffle and louvers block part of the opening — always less than the physical size. Vent makers publish it in square inches per linear foot, and it is what you size ventilation against, not the gross opening.
Why split intake and exhaust evenly?
Balanced ventilation makes air flow up and out: cool intake at the soffits, warm exhaust at the ridge. If exhaust exceeds intake, a ridge vent can pull air back down through itself instead of from the soffits, so codes want at least as much low intake as high exhaust.
When do I use 1:150 versus 1:300?
The 1:300 ratio is allowed when a Class-I vapor retarder is on the warm side of the ceiling and intake and exhaust are balanced; otherwise the more conservative 1:150 applies. Doubling the ratio halves the vent you must install, so it matters.
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Estimates only — add for waste and confirm every load-bearing decision against local building codes and a design professional.