Ridge Vent Calculator

Linear Feet and Sections of Ridge Vent for Your Attic

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. Balanced ventilation needs 1 square foot of net free area per 300 square feet of attic, half at the soffit and half at the ridge.

Ridge vent needed by attic size

Linear feet of ridge vent for the exhaust half at 1/300
Attic floor	Exhaust NFA	At 12 sq in/ft	At 18 sq in/ft
1,000 sq ft	240 sq in	20 ft	13.3 ft
1,500 sq ft	360 sq in	30 ft	20 ft
2,000 sq ft	480 sq in	40 ft	26.7 ft
2,500 sq ft	600 sq in	50 ft	33.3 ft
3,000 sq ft	720 sq in	60 ft	40 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.

Common ridge vent mistakes

Mixing exhaust types. Box vents, gable vents or turbines on the same attic short-circuit the ridge vent and pull air from the nearest opening instead of the soffit.
Exhaust larger than intake. The vent then draws conditioned air out of the house, and weather in through the vent.
Cutting the slot too narrow or into the rafters at the ends.
Using shingle nails. Ridge vents need the longer nails the manufacturer specifies to reach through the vent into the deck.

Frequently asked questions

How much ridge vent do I need?

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.

Can you have too much ridge 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.

How long is a piece of ridge vent?

Most shingle-over ridge vents come in 4 ft sections; roll vents come in 20 to 50 ft rolls.

Does ridge vent work on a hip roof?

Yes, but the short ridge often cannot provide enough exhaust. Hip vents or low-profile exhaust vents near the ridge make up the difference.