As an Amazon Associate I earn from qualifying purchases.
Attic sq ft ÷ 300 × 144 for balanced vents (÷ 150 otherwise), half of it as exhaust, divided by the turbine's net free area — about 113 sq in for a 12 in turbine. A 1,500 sq ft attic: 360 sq in of exhaust, 4 turbines, plus 360 sq in of soffit intake.
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 about half of it in the turbines as exhaust, no more than 3 ft below the ridge. Then divide by the turbine's net free area: about 113 sq in for a 12 in turbine. A 1,500 sq ft attic at 1/300 needs 720 sq in in total and 360 sq in of exhaust, so 360 ÷ 113 = 3.2, rounded up to four turbines, plus 360 sq in of soffit intake (14 rectangular 4 × 16 in vents). At the 1/150 default it is seven turbines. Fixr puts a turbine at $65–$250 installed, so four cost $260–$1,000. Don't add turbines to a roof with a ridge vent: mixed exhausts short-circuit each other.
Short answer: attic floor ÷ 300 (balanced vents) or ÷ 150 × 144 = square inches of vent area; put half in the turbines and divide by the turbine's net free area, about 113 sq in for a 12 in turbine. A 1,500 sq ft attic needs 360 sq in of exhaust at 1/300 — four turbines — and the same 360 sq in as soffit intake.
| Attic | Exhaust at 1/300 | Turbines | Exhaust at 1/150 | Turbines |
|---|---|---|---|---|
| 800 sq ft | 192 sq in | 2 | 384 sq in | 4 |
| 1,000 sq ft | 240 sq in | 3 | 480 sq in | 5 |
| 1,200 sq ft | 288 sq in | 3 | 576 sq in | 6 |
| 1,500 sq ft | 360 sq in | 4 | 720 sq in | 7 |
| 2,000 sq ft | 480 sq in | 5 | 960 sq in | 9 |
| 2,500 sq ft | 600 sq in | 6 | 1200 sq in | 11 |
| 3,000 sq ft | 720 sq in | 7 | 1440 sq in | 13 |
Fixr's roof vent guide (January 2025) puts a turbine at $65–$250 per vent installed, so four turbines cost $260–$1,000.
Work out the exhaust net free area and divide by the turbine's rating. Under IRC R806.2 an attic needs 1 sq ft of vent area per 150 sq ft of floor, or per 300 sq ft when intake and exhaust are balanced; multiply by 144 for square inches and put about half of it in the turbines. A 1,500 sq ft attic at 1/300 needs 720 sq in in total, 360 sq in of exhaust — four 12 in turbines at about 113 sq in each. At 1/150 it would be seven.
A 12 in turbine rated at about 113 sq in of net free area carries the exhaust half for roughly 470 sq ft of attic floor at the 1/300 ratio (113 ÷ 144 × 300 × 2), or about 235 sq ft at 1/150. Read the net free area printed on the turbine you buy — it varies by make and throat size.
Yes, if the intake does not keep up. Turbines pull air out of the attic; if the soffits cannot let the same amount in, they draw conditioned air up from the house through ceiling gaps and light fixtures, or pull rain and snow in through the other roof vents. Keep the intake at least equal to the exhaust.
It is not recommended. Two kinds of exhaust on one attic space short-circuit each other: the turbine draws its air in through the nearby ridge vent instead of from the soffits, so the lower attic stops ventilating. Use one exhaust type per attic, with soffit intake.
High on the roof, near the ridge — the IRC's 1/300 exception wants the exhaust no more than 3 ft below the ridge — and spread evenly along it so each part of the attic is drawn on. Put them on the back slope where you can, centered between rafters or trusses, and keep them clear of valleys, chimneys and the hips.
Comparing exhaust options? The ridge vent calculator sizes a continuous ridge vent instead.
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.