Calculators

Attic ventilation calculator

Safety verdict: Safe from the ground attic floor area is measured from inside

Vented attic floor area (sq ft)The space being ventilated

Ratio

Required net free ventilating area from attic floor area, at the IRC 1/150 baseline or the 1/300 exception, split into intake and exhaust.

The formula

net free area (sq in) = attic sq ft / ratio x 144 ; split 50% intake / 50% exhaust

IRC R806.2 sets the minimum net free ventilating area at 1/150 of the vented space.
The 1/300 reduction is an EXCEPTION requiring BOTH conditions at once: a Class I or II vapor retarder on the warm-in-winter side of the ceiling in Climate Zones 6, 7 and 8; and 40 to 50 percent of the required area provided by ventilators in the upper portion of the attic, not more than 3 feet below the ridge. Choosing 1/300 here asserts that both are true.
Net free area is the actual open area of a vent after its screen and louvers, not the size of the hole. Manufacturers publish it per unit or per linear foot.
A 50/50 intake-exhaust split is the conventional target. Exhaust without matching intake short-circuits and does not ventilate the attic.
Adopted IRC edition varies by state and often by county.