Vents are rated by net free area, the open space that lets air pass through. It is smaller than the vent's outside size, and screens and louvers shrink it further. Match the calculator's total against the net free area printed on each vent you buy.
How to use this calculator
- Measure the attic floor in feet, wall to wall at the ceiling joists. For an L-shaped attic, run each rectangle separately and add the net free vent area results.
- Leave the ratio on the basic option unless the reduced option's conditions fit your roof, as described below.
- Read the total in square inches. The square feet figure is the same area in other units, so use it to check your arithmetic.
- Use the even split as your starting plan: intake at the soffits, exhaust high on the roof.
- Check each vent's printed net free area before you buy, and confirm the plan with your building department, since local rules can differ.
Worked examples
Ranch house attic
Attic: 48 × 32 ft · Ventilation ratio: 1/150 (basic rule)
- Net free vent area
- 1,475 sq in
- Intake (low)
- 737 sq in
- Exhaust (high)
- 737 sq in
- Net free vent area
- 10.24 sq ft
- Attic floor area
- 1,536 sq ft
Cape Cod attic on the reduced option
Attic: 28 × 24 ft · Ventilation ratio: 1/300 (40 to 50% of the vent area high on the roof)
- Net free vent area
- 323 sq in
- Intake (low)
- 161 sq in
- Exhaust (high)
- 161 sq in
- Net free vent area
- 2.24 sq ft
- Attic floor area
- 672 sq ft
Long narrow attic over an addition
Attic: 60 × 16 ft · Ventilation ratio: 1/150 (basic rule)
- Net free vent area
- 922 sq in
- Intake (low)
- 461 sq in
- Exhaust (high)
- 461 sq in
- Net free vent area
- 6.40 sq ft
- Attic floor area
- 960 sq ft
Small cottage attic
Attic: 20 × 14 ft · Ventilation ratio: 1/300 (40 to 50% of the vent area high on the roof)
- Net free vent area
- 134 sq in
- Intake (low)
- 67 sq in
- Exhaust (high)
- 67 sq in
- Net free vent area
- 0.93 sq ft
- Attic floor area
- 280 sq ft
Net free vent area in square inches for common attic sizes, at the basic and reduced ratios
| Attic | Ventilation ratio: 1/150 | Ventilation ratio: 1/300 |
|---|---|---|
| 20 × 14 ft | 269 | 134 |
| 30 × 24 ft | 691 | 346 |
| 40 × 30 ft | 1,152 | 576 |
| 50 × 36 ft | 1,728 | 864 |
Each cell: Net free vent area (sq in). Inputs not in the table use the calculator's starting values.
How the calculation works
The calculator multiplies the length by the width to get the floor area, divides that by the ratio, and converts the result to square inches. The total is then split into an intake (low) share and an exhaust (high) share of equal size.
The residential building code's basic rule asks for a square foot of net free vent area for every 150 sq ft of attic. The reduced rule lets that drop to a square foot for every 300 sq ft of attic when 40 to 50 percent of the vent area sits high on the roof, no more than 3 ft below the ridge, with the rest low at the eaves. In the coldest climate zones the code can also require a vapor retarder on the warm side of the ceiling before the reduced rule applies. Local codes can differ, and the building department has the final word.
Before you pick the reduced option, measure where the high vents would sit. If you need the roof's slope for that, the roof pitch calculator is the place to start.
Net free area and the vents you buy
Net free area is the open area a vent passes air through, not its outside size. Screens and louvers reduce it further, so vents of the same outside size can rate very differently.
Intake starts at the soffits, the underside of the eaves. Insulation pushed against the soffit vents blocks that airflow, so keep it back from them. A plan with enough area on paper does nothing if air cannot reach the vents.
If the roof covering is being replaced at the same time, plan that material with the roofing calculator.
Mistakes to avoid
- Using a vent's outside size instead of its net free area. The printed rating is the figure to compare.
- Choosing the reduced option without checking the roof. The high vents have to meet the condition before you pick it.
- Letting insulation cover the soffit vents. A blocked intake adds nothing, however much area the plan shows.
- Treating a divided attic as if it were open throughout. Ask the building department how a partitioned attic should be counted.
What types of attic vents are there, and where do they go?
Intake vents sit low, usually in the soffits. Exhaust vents sit high on the roof. Common exhaust types are ridge vents along the peak, box vents on the roof surface, and gable vents in the end walls. Air comes in low, rises as it warms, and leaves high. For the reduced option, the high vents must sit no more than 3 ft below the ridge, so check that before you choose a type.
How much net free area does each type of vent give?
Net free area depends on the product, so use the number printed on your vent's label or data sheet. A common box vent, for example, gives about 50 square inches.
Common questions
How many roof vents do I need?
The calculator gives a total area, not a vent count. Divide the net free vent area by the rating printed on the vent you choose, and round up. Work out the intake and exhaust totals separately.
What is the code for attic ventilation?
The calculator follows the residential building code's rule, which sets a basic ratio and a reduced option for certain vent layouts. Local amendments can change it, so ask your building department for the ventilation section of the code they enforce.
What does the reduced rule for roof ventilation mean?
It allows less total vent area when part of that area sits high on the roof. The reduced option applies when 40 to 50 percent of the vent area is high, no more than 3 ft below the ridge, and the rest is low at the eaves. Cold climates can add a vapor retarder requirement, so confirm the layout with the building department before you rely on it.
How do I calculate attic vent size?
Multiply the attic's length by its width, then divide by the ratio you chose. The calculator shows the required net free area in square inches and square feet. Pick vents whose printed net free areas add up to that total, not vents picked by outside size.
Can I just add more vents to fix my attic airflow?
More is not always better. Extra exhaust vents high on the roof can pull air in through each other instead of through the soffits. Aim for the total from the calculator, split between low intake and high exhaust, and keep the soffits clear.
What are the signs of poor attic ventilation?
Common signs are a very hot attic in summer, frost or damp insulation in winter, musty smells, and ice dams at the eaves. Shingles that age early can also point to trapped heat. If you see these signs, measure the attic and compare your vents to the calculator's total.
Are powered attic fans worth it?
A fan does not replace the vent area the calculator gives you. A fan with too little intake can pull air up from the rooms below instead of in through the soffits. Plan the intake and exhaust vent area first, and ask a roofer or your building department before you add a fan.
What are the attic ventilation rules in California (or my state)?
The model residential code starts at 150 sq ft of attic per square foot of vent. It allows 300 sq ft of attic only if every listed condition is met, including 40 to 50 percent of the vent area high on the roof. California and many cities adopt their own edition or amendments, so check with your local building department.