Roof Ventilation CFM Calculator
Find out exactly how much attic ventilation your Tulsa-area home needs. Enter your attic square footage, roof pitch, and current vents — get required net free area per the 1/300 rule, CFM airflow, and how many ridge, soffit, or box vents to add. Built from IRC R806.2 ventilation requirements and Oklahoma climate data.
Ventilation CFM Calculator
Adjust the inputs — required net free area, CFM, and vents to add update instantly using the 1/300 ventilation rule.
How To Read Your Results
Net Free Area (NFA)
The 1/300 rule requires 1 sq ft of net free vent area for every 300 sq ft of attic floor — about 960 sq in for a 2,000 sq ft attic. NFA is the open area of a vent after screens and louvers, not its overall size. That's the number that matters for code and airflow.
CFM Airflow
Multiply your required NFA (in sq ft) by 750 ft/min — the standard design velocity for natural attic ventilation. A 2,000 sq ft attic needs about 5,000 CFM of airflow. Steeper pitches add attic volume, so the calculator adjusts your CFM target above 4/12.
Balance Intake & Exhaust
Ventilation only works when intake and exhaust are balanced — roughly 50/50. Soffit vents bring cool air in at the eaves; ridge and box vents let hot air escape at the peak. If you're short on either side, adding one type without the other won't fix the attic. Get a free roof estimate from Proof Construction. Explore all roofing solutions from Proof Construction.
Frequently Asked Questions
Quick Answers
How much net free area does a ridge, soffit, or box vent provide? Typical installed values: a shingle-over ridge vent provides about 18 sq in of net free area per linear foot, a 4x16 aluminum soffit vent provides about 56 sq in, and a standard 8x16 roof louver (box vent) provides about 50 sq in. Always verify the manufacturer's stamped NFA rating - cheap vents often deliver less than their nominal size.
What is the 1/300 roof ventilation rule? The 1/300 rule (IRC R806.2) says your attic needs at least 1 square foot of net free vent area for every 300 square feet of attic floor space, split roughly 50/50 between intake (soffit) and exhaust (ridge or box) vents. A 2,000 sq ft attic therefore needs about 6.7 sq ft - roughly 960 sq in - of total vent area. Homes without a vapor barrier, or with cathedral ceilings, often need the tighter 1/150 rule instead.
How many CFM does my attic ventilation need? Multiply your required net free area (in square feet) by 750 feet per minute - the standard air velocity used for natural attic ventilation design. A 2,000 sq ft attic on the 1/300 rule needs about 6.7 sq ft of vent area, which moves roughly 5,000 CFM. Steeper pitches add attic volume, so this calculator bumps the CFM target slightly for pitches above 4/12.
Why does attic ventilation matter in Tulsa's climate? Tulsa summers regularly top 100 degrees, and an unvented attic can hit 150 degrees or more - baking shingles from underneath and shortening their life. Proper ventilation also exhausts moisture in winter, preventing condensation, mold, and ice dams. Balanced intake and exhaust is the key: soffit intake paired with ridge or box exhaust.
Can Proof Construction add or fix my attic ventilation? Yes. We install ridge, box, and soffit ventilation on Tulsa-area homes and can balance an attic that is short on intake or exhaust. Call (918) 734-4444 or use our contact page to book a free inspection and get a flat quote - no pressure, no obligation.
