Understanding your result
The headline is the number of box vents, with their rating, the exhaust they meet and the spacing. The stats show the exhaust needed, the vents per plane and the area delivered.
The exhaust comes from the attic ventilation calculator; the intake to match it from the soffit vent calculator. If the roof has a ridge long enough, the ridge vent calculator is the alternative.
How we calculate this
The spacing figure assumes the vents sit in one row along the ridge on each plane you choose. On a hip roof the “ridge” for spacing is the short ridge plus the upper part of the hips, so spread the vents along the top of each plane rather than bunching them at the centre: the aim is that no part of the attic is far from an exhaust, because air near a single vent moves while air at the far end of the attic stays still. Two rows are never needed on a house; if one row cannot fit the count, the vent rating is too low or the attic needs a ridge vent.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Rule | 1:300, half as exhaust | IRC R806.2 |
| Vent rating | 50 sq in | Common box vent (50 to 61) |
| Placement | about 2 ft below the ridge | Vent makers’ instructions |
Assumptions last reviewed October 7, 2026.
The calculator does not flash the vents or check rafter positions. The ventilation table lists requirements by attic size, and the ventilation guide covers mixing exhaust types.
Two worked examples
A 1,500 square foot attic on one plane
1:300, 50 square inch vents, 44-foot ridge, one plane.
- Exhaust: 360 sq in; vents: 360 ÷ 50 = 7.2, so 8, about 5.5 ft apart; delivered 400 sq in
The same attic under 1:150 on two planes
61 square inch vents, two planes.
- Exhaust: 720 sq in; vents: 720 ÷ 61 = 11.8, so 12, 6 per plane about 7.3 ft apart
Twelve holes in the roof: a ridge vent would deliver the same in one run.
Where to find your inputs
Attic floor area and rule. From the attic ventilation calculator.
Vent rating. On the product.
Ridge length and planes. From the plan.
Common mistakes
- Box vents with a ridge vent. One exhaust type.
- No intake. Soffits first.
- All at one end. Spread them.
- Too low. Exhaust belongs near the ridge.
- Over a rafter. Cut between rafters.
- Poor flashing. Upper shingles over the base.
Questions people ask
- How many roof vents do I need?
- Divide the exhaust net free area by each vent's rating. A 1,500 square foot attic under the 1:300 rule needs 360 square inches of exhaust; at 50 square inches per box vent that is 8 vents.
- How much net free area does a box vent have?
- Most low-profile and slant-back box vents are rated at 50 to 61 square inches. The rating is on the product.
- Where should box vents go?
- In a row about 2 feet below the ridge, evenly spaced, on the rear or least visible plane, each between rafters. Spreading them across the attic's length draws the whole space.
- Box vents or ridge vent?
- A ridge vent gives continuous exhaust along the highest point and looks cleaner; box vents suit hip roofs and short ridges and are easier to add to an existing roof. Do not use both.
- Can box vents leak?
- Like any penetration, if poorly flashed. The vent's base goes under the shingles above and over those below, sealed at the nails; quality vents have baffles against wind-driven rain.
- How big a hole for a box vent?
- The size on the vent's template, often 8 to 10 inches square, cut between rafters through the sheathing only.