Attic Ventilation Calculator (Net Free Area, Ridge & Soffit Vents)
How much attic ventilation you need by the 1/150 and 1/300 rules (Florida Building Code, Residential R806.2), split into intake and exhaust, and how many vents that is for your vent products.
Formula
Total net free area (sq in) = attic floor area (sq ft) × 144 ÷ 150 (or ÷ 300 under the balanced exception). Exhaust NFA = total × high share; intake NFA = the rest. Vents = ⌈NFA ÷ the net free area of one vent (or one foot of continuous vent)⌉.
Worked example
Inputs: Attic floor area (vented space) = 1500 sq ft; Ventilation rule = 150; Share of vent area that is high (exhaust) = 50 %; Exhaust vent net free area per unit or per ft of ridge vent (from the label) = 18 sq in; Intake vent net free area per unit or per ft of soffit vent (from the label) = 9 sq in.
| Total nfa sq in | 1,440 |
|---|---|
| Total nfa sq ft | 10 |
| Exhaust nfa sq in | 720 |
| Intake nfa sq in | 720 |
| Exhaust units | 40 |
| Intake units | 80 |
Balance intake and exhaust; intake goes low (soffits), exhaust high (ridge or roof vents).
FAQ
How much attic ventilation do I need?
R806.2 requires a net free vent area of 1/150 of the vented space. A 1,500 sq ft attic needs 1,500 × 144 ÷ 150 = 1,440 sq in (10 sq ft). Under the 1/300 exception it is 720 sq in.
When can I use 1/300?
When 40 to 50 percent of the required vent area is in the upper part of the attic, no more than 3 ft below the ridge, and the rest is in the bottom third of the attic (Florida Building Code, Residential R806.2 exception).
How many feet of ridge vent do I need?
Divide the exhaust area by the net free area per foot printed on the ridge vent. With 720 sq in of exhaust and a vent rated 18 sq in per foot, that is 40 ft. The 18 and 9 sq in defaults are placeholders, not typical values: use your product's label.
What is net free area?
The open area air can actually pass through after screens and louvers, which is smaller than the vent's outside size. Vent makers print it on the package.
Does every attic need vents?
No. R806.5 allows unvented attics that meet all of its conditions. This calculator is for vented attics; ask your building department which applies.
Sources
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Last reviewed 2026-10-20.