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Attic floor area ÷ 300 (balanced vents) or ÷ 150, × 144, and at least half of it as intake. A 1,500 sq ft attic needs 360 sq in of intake at 1/300: 14 rectangular 4 × 16 in vents at about 26 sq in each, or 40 ft of continuous soffit strip. Use the NFA printed on your vent.
Divide the attic floor area by 150 — or by 300 when intake and exhaust are balanced, per IRC R806.2 — multiply by 144 for square inches of net free area, and put at least half of it in the soffits as intake. A 1,500 sq ft attic at 1/300 needs 720 sq in in total and 360 sq in of intake: 14 rectangular 4 × 16 in soffit vents at about 26 sq in each, 40 ft of continuous soffit strip at 9 sq in per foot, or 36 sq ft of fully vented vinyl soffit. At the 1/150 default it is twice that: 28 rectangular vents, about 43 in apart on 100 ft of eave. Always divide by the net free area printed on the vent you buy.
IRC R806.2 sets attic ventilation at 1 sq ft of net free area per 150 sq ft of attic floor, or 1 per 300 when 40–50% of it is exhaust within 3 ft of the ridge and the rest is intake low (plus a vapor retarder in climate zones 6–8). Soffit vents are the intake: at least half the total, up to 60%. Divide by the net free area printed on the vent you buy.
| Attic | Intake at 1/300 | 4×16 vents (26 sq in) | Strip (9 sq in/ft) | Intake at 1/150 | 4×16 vents |
|---|---|---|---|---|---|
| 800 sq ft | 192 sq in | 8 | 22 ft | 384 sq in | 15 |
| 1,000 sq ft | 240 sq in | 10 | 27 ft | 480 sq in | 19 |
| 1,200 sq ft | 288 sq in | 12 | 32 ft | 576 sq in | 23 |
| 1,500 sq ft | 360 sq in | 14 | 40 ft | 720 sq in | 28 |
| 2,000 sq ft | 480 sq in | 19 | 54 ft | 960 sq in | 37 |
| 2,500 sq ft | 600 sq in | 24 | 67 ft | 1,200 sq in | 47 |
| 3,000 sq ft | 720 sq in | 28 | 80 ft | 1,440 sq in | 56 |
| Intake product | Typical NFA | Note |
|---|---|---|
| Continuous soffit vent strip | 9 sq in per linear foot | Perforated strip; varies widely with the perforation pattern |
| Vented vinyl soffit panel | 10 sq in per square foot | Fully vented panel; centre-vent panels give far less |
| Rectangular soffit vent 4x16 in | 26 sq in each | Common retrofit size |
| Round soffit vent 2 in | 3 sq in each | Small; many are needed |
| Round soffit vent 3 in | 7 sq in each |
Divide the attic floor area by 150 (or by 300 when the vents are balanced, per IRC R806.2), multiply by 144 for square inches, and put at least half of that low as intake. A 1,500 sq ft attic at 1/300 needs 360 sq in of intake: 14 rectangular 4 × 16 in vents at about 26 sq in each, or 40 ft of continuous soffit strip at 9 sq in per foot.
Match the soffit intake to at least the ridge vent's net free area. Ridge vent is commonly rated about 18 sq in per foot, so 30 ft of ridge vent (540 sq in) needs 540 sq in of intake: about 21 rectangular 4 × 16 in soffit vents or 60 ft of continuous soffit strip. Too little intake and the ridge vent pulls air from the house instead.
Spread them evenly along every eave so each rafter bay gets air: divide the eave length by the number of vents. Twenty-eight 4 × 16 in vents on 100 ft of eave land about 43 in apart. Continuous strip or fully vented panels avoid dead bays altogether, and rafter baffles keep insulation from blocking the opening.
Not really. Extra intake is the safe direction: the IRC exception allows exhaust of only 40–50% of the total, so 50–60% intake is normal. The real problem is the reverse — more exhaust than intake — which depressurises the attic and draws conditioned air up through the ceiling.
Read the net free area printed on the vent, not its outside size. A 4 × 16 in louvered vent is about 26 sq in NFA, a 2 in round plug only about 3 sq in. Continuous strip and fully vented vinyl panels give the most intake per foot of eave with the fewest holes to cut.
The whole attic at once: the roof vent calculator gives intake and exhaust together.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.