Attic Ventilation Calculator â How Much Venting Do I Need?
Enter your attic floor area to get total vent area, the intake/exhaust split, and CFM. No sign-up, no data stored.
Ventilation needed
How attic ventilation is calculated
- Total vent area = attic floor area ÷ ratio (150 or 300). A 1,200-sq-ft attic at 1:150 needs 8 sq ft.
- Split: half intake (soffits) and half exhaust (ridge), so each side gets 4 sq ft in that example.
- Vent counts: divide each side's area by the net free area per vent (soffit) or per linear foot (ridge).
- CFM â attic floor area à 0.7, the common sizing target for powered ventilation.
Use net free area (NFA), not gross opening size â screens and louvers reduce actual airflow by 30â50%. If your home has a vapor barrier on the ceiling, the 1:300 ratio is allowed, which halves the vent area.
Attic size â ventilation reference
Total vent area at the 1:150 ratio for common attic sizes.
| Attic area | Total vent | Intake / Exhaust | Ridge vent |
|---|---|---|---|
| 800 sq ft | 5.3 sq ft | 2.7 / 2.7 sq ft | ~21 ft @ 18 sq in/ft |
| 1,000 sq ft | 6.7 sq ft | 3.3 / 3.3 sq ft | ~27 ft |
| 1,200 sq ft | 8 sq ft | 4 / 4 sq ft | ~32 ft |
| 1,500 sq ft | 10 sq ft | 5 / 5 sq ft | ~40 ft |
| 2,000 sq ft | 13.3 sq ft | 6.7 / 6.7 sq ft | ~53 ft |
Last updated: October 2026 · Estimates use the 1:150 rule and 18 sq in of net free area per linear foot of ridge vent. Verify manufacturer NFA ratings before buying.
Attic ventilation FAQ
Provide 1 sq ft of net free vent area per 150 sq ft of attic floor area, split about 50/50 between low intake (soffits) and high exhaust (ridge). With a vapor barrier, codes generally allow 1:300.
Enough to cover half of the total vent area. Most ridge vents provide about 18 sq in of net free area per linear foot, so a 1,200-sq-ft attic needs roughly 32 ft of ridge vent (4 sq ft à 144 ÷ 18).
Match the intake to the exhaust. A typical 4Ã12 soffit vent nets about 36â48 sq in, so a 1,200-sq-ft attic needs roughly 12â16 vents split between both eaves (4 sq ft à 144 ÷ 40 â 14).
No â unbalanced ventilation doesn't work. Ridge vent without soffit intake pulls air from inside the house (or does nothing); soffit vents without ridge exhaust stall. You need both sides roughly equal.
About 0.7 CFM per square foot of attic area is the common sizing target. A 1,200-sq-ft attic wants roughly 840 CFM when using powered exhaust ventilation.
Related calculators & guides
How Much Attic Ventilation Do I Need?
Attic ventilation sizing with the 1/300 rule, net free area (NFA), intake vs exhaust split, and a worked example for a 1,200 sq ft attic.
Read the guide →