Understanding your result
The headline is the number of vents, or the feet of vented soffit, with the type and spacing. The stats give the intake, the eave length and the count or length. The breakdown shows the intake required and how the chosen product delivers it.
The intake figure comes from the attic ventilation calculator, and it should match or exceed the exhaust from the ridge vent calculator. Intake only works with clear bays, which the rafter baffles calculator counts.
How we calculate this
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Rule | 1:300, half as intake | IRC R806.2 |
| Rectangular vent | 56 sq in | A common 16 × 8 in aluminum undereave vent |
| Strip vent | 9 sq in per ft | Common 2-in continuous strip |
| Vented panel | 7 sq in per sq ft | A mid-range vented vinyl soffit; ratings vary |
| Soffit depth | 12 in | Common eave overhang |
Assumptions last reviewed October 7, 2026.
The calculator does not check that each rafter bay has its own vent. The ventilation table lists typical vent ratings, and the ventilation guide explains balance.
Two worked examples
Rectangular vents for a 1,500 square foot attic
1:300, 56 square inch vents, 80 feet of eave.
- Intake: 360 sq in; vents: 360 ÷ 56 = 6.4, so 7, about 11.4 ft apart
Strip vent for a 2,200 square foot attic under 1:150
Intake 1,056 square inches, strip at 9 square inches per foot, 80 feet of eave.
- Strip: 1,056 ÷ 9 = 117 ft, longer than the 80 ft of eave
The eaves cannot carry it; a higher-rated vent or the 1:300 rule with balanced vents is the answer.
Where to find your inputs
Attic floor area and rule. From the attic ventilation calculator.
Vent rating. Printed on the vent or its box.
Eave length. The soffit length on the eave sides.
Common mistakes
- Using the vent size. Use the net free area.
- Trusting vented panels. Check the rating.
- Blocked bays. Fit baffles.
- Vents near the wall. Centre them in the soffit.
- Less intake than exhaust. The house supplies the difference.
- Painting over vents. Paint closes the louvres.
Questions people ask
- How many soffit vents do I need?
- Divide the intake net free area by each vent's rating. A 1,500 square foot attic under the 1:300 rule needs 360 square inches of intake; at 56 square inches per 16 by 8 inch vent that is 7 vents, spaced about 11 feet apart along 80 feet of eave.
- How much intake does continuous soffit strip give?
- Typically about 9 square inches per foot for a 2-inch strip, so 360 square inches needs 40 feet. Check the product; ratings vary.
- Is vented vinyl soffit enough?
- Sometimes not. Vented panels often deliver only 5 to 10 square inches of net free area per square foot of panel, so a 12-inch-deep soffit gives 5 to 10 per foot of eave. Check the rating and compare with the intake needed.
- Where should soffit vents go?
- Evenly along the eaves, in the bays between rafters, centred in the soffit and clear of the wall so the insulation does not cover them. Baffles in each vented bay keep the air path open.
- Can I have too much intake?
- Not in practice; more intake than exhaust keeps the attic slightly pressurised, which is good. Too little intake is the problem.
- Why are my soffit vents not working?
- Usually insulation blocks them where the rafters meet the wall. Rafter baffles fix it; the rafter baffles calculator counts them.