Understanding your result
The headline is the length of ridge vent to install, with the pieces and the net free area it delivers against what the attic needs. The stats give the length needed, the area delivered and the pieces; the breakdown shows the exhaust requirement, the length it takes, the usable ridge and any shortfall.
The two numbers to compare are needed and delivered. If delivered is much larger, check the soffit intake on the soffit vent calculator matches it. If the ridge is too short, the shortfall row says how much exhaust to add with the box vent calculator. The cap shingles over the vent are counted by the ridge cap calculator.
How we calculate this
Half the total net free area is assigned to exhaust, the balance the code’s 1:300 rule assumes and that the attic ventilation calculator uses. The ventilation table lists the exhaust needed for common attic sizes.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Rule | 1:300, half as exhaust | IRC R806.2 |
| Vent rating | 18 sq in per ft | Common shingle-over ridge vent (12 to 20) |
| Piece length | 4 ft | Rigid ridge vent sections |
| Stop short | 6 in at each end | Ridge vent makers’ instructions |
| Nails | 8 per 4 ft piece, 2½ in or longer | Makers’ instructions |
Assumptions last reviewed October 7, 2026.
The calculator does not check the intake, does not handle hip roofs with short ridges beyond flagging the shortfall (hip vents exist for that), and assumes one product along the whole ridge. The guide to the 1:150 rule covers balancing intake and exhaust.
Two worked examples
A 44-foot ridge over a 1,500 square foot attic
1:300 rule, 18 square inches per foot, 4-foot pieces.
- Exhaust: 1,500 ÷ 300 × 144 ÷ 2 = 360 sq in; needed: 20 ft
- Usable ridge: 44 − 1 = 43 ft; 43 ft installed in 11 pieces, delivering 774 sq in
Twice the exhaust needed. Fine with 774 square inches of soffit intake; otherwise shorten the slot.
A short hip-roof ridge
900 square inches of exhaust needed, 30-foot ridge.
- Needed: 50 ft; usable: 29 ft, delivering 522 sq in
- Shortfall: 378 sq in, about eight 50 sq in box vents or hip vents
Where to find your inputs
Attic floor area. The ceiling area under the attic.
Ridge length. From the plan or the ridge cap.
Rating and piece length. On the vent’s packaging.
Common mistakes
- Vent without intake. It draws air in through itself.
- Mixing exhaust types. Close the gable vents.
- Short nails. 2½ inches or longer.
- Slot too narrow. Follow the maker’s width.
- Cutting into the ridge board or trusses. Sheathing only.
- Running to the very end. Stop short of ends and hips.
Questions people ask
- How many feet of ridge vent do I need?
- Divide the exhaust net free area by the vent's rating per foot. A 1,500 square foot attic under the 1:300 rule needs 720 square inches, half of it (360) as exhaust; at 18 square inches per foot that is 20 feet of vent.
- Should ridge vent run the whole ridge?
- Usually, yes, for looks and to draw the whole attic, stopping about 6 inches short of each end and of any hip. That often gives more exhaust than the rule needs, which is fine provided the soffit intake at least matches it.
- What is a typical ridge vent rating?
- Most shingle-over ridge vents are rated between 12 and 20 square inches of net free area per linear foot; 18 is common. The rating is printed on the product and is the figure to use.
- How wide is the slot for a ridge vent?
- About 1 inch each side of the ridge on a truss roof (a 2-inch slot), or 1½ inches each side with a ridge board (3 inches), cut through the sheathing only. The vent maker's instructions give the exact width.
- Can I use a ridge vent with gable vents?
- Not recommended. The gable vents become intakes for the ridge vent and the soffits stop drawing. Close the gable vents when a ridge vent goes on.
- What nails hold a ridge vent?
- Long ones: the nails go through the cap shingles and the vent into the deck, so 2½ inches or longer. Short nails are the commonest cause of ridge vents and caps blowing off.