Free Tool
Ridge Vent & Attic Ventilation Calculator
Enter attic square footage to estimate the required net free ventilation area and ridge vent length.
Use the attic floor footprint, not the roof surface area — for a multi-section attic, add each section together first.
Ventilation ratio
A vapor retarder is installed on the ceiling's warm-in-winter side — the most common modern setup with ridge + soffit venting
Enter your attic’s floor area above to see your results.
This assumes a roughly even 50/50 split between ridge (exhaust) and soffit (intake) venting, within code’s allowed 40-50% upper range. Treat this as a planning estimate — actual vent product coverage and local code requirements should be confirmed before ordering.
What is Net Free Area (NFA)?
Net Free Area is the actual open, unobstructed area of a vent product — not its physical size. A vent’s frame, mesh, and baffles all take up space, so its NFA is always smaller than its overall dimensions. Building codes and every ventilation calculation, including this one, work in NFA, not physical vent size.
The 1:150 vs. 1:300 rule
The International Residential Code (IRC R806.2) sets the minimum attic ventilation at 1 square foot of NFA for every 150 square feet of attic floor space. That requirement relaxes to 1:300 when ventilation is balanced — 40-50% of the required area placed high (ridge or upper vents) with the rest low (soffit or eave vents) — and a vapor retarder is installed on the ceiling’s warm-in-winter side. This calculator always splits ridge and soffit roughly 50/50, which already satisfies the balanced-venting half of that rule, so the ratio toggle above is really asking about the vapor retarder: pick 1:300 if one’s installed, 1:150 if it isn’t or you’re not sure.
Why balanced intake and exhaust matters
A ridge vent works by pulling outside air in through the soffits and pushing warm attic air out at the top. Without enough intake area to match it, a ridge vent can end up pulling air from the wrong places — gaps around the ceiling, for instance — instead of the soffits, which defeats the purpose and can pull conditioned air out of the house below. That’s why code requires intake and exhaust to be roughly balanced, not just a large total number.
Reference table (1:300, default NFA)
Common attic sizes at the standard 1:300 ratio, using 18 in²/ft ridge vent and 9 in²/ft soffit vent — the same defaults this calculator uses.
| Attic area | Total NFA | Ridge vent | Soffit vent |
|---|---|---|---|
| 500 sq ft | 240 in² | 6.7 ft | 13.3 ft |
| 1,000 sq ft | 480 in² | 13.3 ft | 26.7 ft |
| 1,500 sq ft | 720 in² | 20.0 ft | 40.0 ft |
| 2,000 sq ft | 960 in² | 26.7 ft | 53.3 ft |
| 2,500 sq ft | 1200 in² | 33.3 ft | 66.7 ft |
| 3,000 sq ft | 1440 in² | 40.0 ft | 80.0 ft |
Worked example
An 1,800 sq ft attic, balanced ventilation (1:300), default NFA figures:
- 1. Total NFA: (1,800 / 300) × 144 = 864 in²
- 2. Split 50/50: 432 in² exhaust (ridge), 432 in² intake (soffit)
- 3. Ridge vent length: 432 / 18 = 24 ft
- 4. Soffit vent length: 432 / 9 = 48 ft
Frequently asked questions
What is Net Free Area (NFA)?
The actual open, unobstructed area of a vent product — not its physical size. A vent's frame, mesh, and baffles all take up space, so a vent's NFA is always smaller than its overall dimensions. It's what building codes and vent calculations actually measure.
Do I need 1:150 or 1:300 attic ventilation?
Code allows 1:300 when ventilation is balanced between ridge and soffit (which this calculator always assumes) and a vapor retarder is installed on the ceiling's warm-in-winter side. Without a vapor retarder, use 1:150 instead.
Why does intake need to roughly match exhaust?
Ridge vents rely on soffit vents pulling in outside air — without enough intake, a ridge vent can end up pulling air from the wrong places (like gaps around the ceiling) instead of the soffits, which defeats the point. Code requires 40-50% of the total ventilation area to sit high (ridge) and the rest low (soffit).
Can I use my exact ridge or soffit vent product's NFA instead of the default?
Yes — NFA per linear foot varies meaningfully between manufacturers and products, commonly 12.5-20 in²/ft for ridge vents. Check the product's packaging or spec sheet and enter it directly instead of the default.
What happens if an attic isn't ventilated enough?
Heat and moisture build up. In summer that means a hotter attic pushing more heat into the living space below; in winter, trapped moisture can condense and contribute to mold, wood rot, and ice dams along the eaves.