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The Ridge Vent Calculator works out how much ridge vent an attic needs and whether your ridge is long enough to provide it. Divide the attic floor area by 300 for balanced ventilation (or by 150 when there is no vapor retarder and no balanced intake), multiply by 144 to get square inches of net free area, and put half of it at the ridge. Divide that exhaust figure by the vent's rated net free area per linear foot — most shingle-over ridge vents are rated between 12 and 18 square inches per foot — to get the feet of vent required. A 1,500 square foot attic needs 720 square inches in total and 360 at the ridge, which is 20 feet of an 18 sq in per foot vent. Usable ridge is the ridge length less about 6 inches at each end. On a hip roof the ridge is short and often cannot meet the requirement, and the calculator shows the shortfall so you can plan hip vents or low-profile exhaust vents instead.
Ridge vent is sized from the attic floor area, not the roof area. Code (IRC R806.2) needs 1 square foot of net free area per 150 square feet of attic, half at the soffit and half at the ridge. 1/150 is the IRC R806.2 default. It halves to 1/300 only under the code exception: 40–50% of the vent area is exhaust within 3 feet of the ridge with the rest as low intake, plus, in climate zones 6–8, a Class I or II vapour retarder on the warm side of the ceiling. A vapour retarder on its own does not qualify.
| Attic floor | Exhaust NFA | At 12 sq in/ft | At 18 sq in/ft |
|---|---|---|---|
| 1,000 sq ft | 480 sq in | 40 ft | 26.7 ft |
| 1,500 sq ft | 720 sq in | 60 ft | 40 ft |
| 2,000 sq ft | 960 sq in | 80 ft | 53.3 ft |
| 2,500 sq ft | 1200 sq in | 100 ft | 66.7 ft |
| 3,000 sq ft | 1440 sq in | 120 ft | 80 ft |
Usable ridge is the ridge length less about 6 inches at each end and at every hip junction. Most shingle-over vents come in 4 ft sections.
Attic floor area ÷ 300 × 144 gives the total net free area in square inches; half of it goes at the ridge. Divide by the vent rating per linear foot. A 1,500 sq ft attic needs 360 sq in of exhaust, or 20 ft of an 18 sq in per foot vent.
Only if intake is short. Venting the full usable ridge is fine as long as the soffit provides at least as much net free area as the ridge.
Most shingle-over ridge vents come in 4 ft sections; roll vents come in 20 to 50 ft rolls.
Yes, but the short ridge often cannot provide enough exhaust. Hip vents or low-profile exhaust vents near the ridge make up the difference.
Choosing a ridge vent? See our roof vent buying guide.
Comparing a ridge vent with a powered fan? See the attic fan installation cost calculator.
What will the roofer charge? A ridge vent runs $300–$650 installed — see the roof vent installation cost calculator.
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.