Ridge Vent Calculator — How Much Ridge Vent Do I Need?
Calculate the length of ridge vent needed for proper attic ventilation using the 1/150 or 1/300 rule based on your attic floor area.
How to Use This Calculator
- Enter your attic floor dimensions (length and width)
- Enter the total ridge length of your roof
- Select the ventilation rule (1/150 is standard for most attics)
- Select your ridge vent product's NFA rating — check the product packaging
- Click Calculate to get the required linear feet of ridge vent
How the Calculation Works
// Required net free area (NFA):
required_nfa_sqin = (attic_sqft / ventilation_rule) × 144
// Linear feet of ridge vent needed:
ridge_vent_ft = ⌈required_nfa_sqin / nfa_per_ft⌉
// Maximum allowed:
max_ridge_vent = ridge_length − 2 // leave 12" clearance at each end Attic ventilation is governed by the 1/150 rule: one square foot of net free area for every 150 sq ft of attic floor. Ridge vents are rated by NFA per linear foot. Dividing required NFA by the product's NFA rating gives the feet of vent required.
Variables:
- attic_sqft: Floor area of the attic in square feet
- nfa_per_ft: Net free area of the ridge vent in sq inches per linear foot
- ridge_length: Total length of the roof ridge in feet
Pro Tips
- The 1/150 rule: 1 sq ft of net free area (NFA) per 150 sq ft of attic floor
- With a vapor barrier on the attic floor, the 1/300 rule applies instead
- Ridge vents work best paired with soffit intake vents — always balance intake and exhaust
- Never let insulation block the eave airflow — use rafter baffles to maintain a clear 2" channel
- Most high-performance ridge vents provide 18 sq in of NFA per linear foot
Recommended Products
Owens Corning VentSure Ridge Vent — 4 ft Section
Hip and ridge vent with filter for weather protection.
View on Amazon →Air Vent ShingleVent II Ridge Vent — 4 ft
High-flow shingle-over ridge vent, 18 NFA per ft.
View on Amazon →Coil Roofing Nails — 1-3/4" x 120-count
Galvanized coil nails for ridge cap and vent fastening.
View on Amazon →DeWalt Reciprocating Saw Blade — 6" (5-pack)
Cut ridge slot in OSB sheathing for vent installation.
View on Amazon →Frequently Asked Questions
How much ridge vent do I need for a 2,000 sq ft attic?
Using 1/150: need 13.3 sq ft NFA = 1,920 sq in. With 18 sq in/ft vent, you need about 107 linear feet of ridge vent.
Can I have too much ridge vent?
Yes. Excess ridge vent without enough soffit intake creates reverse airflow, drawing outside air in through the ridge. Balance intake and exhaust venting equally.
Should ridge vents go the full length of the ridge?
Install as much as possible, but leave at least 12 inches of solid decking at each end of the ridge for structural integrity.
What is net free area (NFA)?
Net free area is the actual open space in a vent after accounting for screens and baffles, measured in square inches. Higher NFA means more airflow per linear foot.