Attic Ventilation Calculator

Follow these steps to size the vents for an attic. The whole process takes less than a minute.

1. Enter the attic floor area in square feet. Measure the ceiling area of the rooms below the attic, or multiply the length of the attic by its width.
2. Choose the ventilation ratio. Pick 1/150 for the standard requirement, or 1/300 when intake and exhaust are balanced and the code conditions for the lower ratio are met.
3. Select the exhaust vent type: a ridge vent, gable vents or roof louvers. Exhaust vents sit at or near the top of the attic.
4. Select the intake vent type: continuous soffit vent or individual soffit vents. Intake vents sit low, along the eaves.
5. Read the results. The total net free area, the intake share, and the number of exhaust and intake vents update right away.

Treat the output as a planning figure. Always check the net free area printed on the label of the exact product you plan to buy.

Attic ventilation is the controlled movement of outdoor air through the attic space. Air enters through intake vents low along the eaves, rises as it warms, and leaves through exhaust vents at or near the ridge. This steady flow carries away moist air and reduces the heat that builds up under the roof deck.

Good attic ventilation matters for several reasons.

• Moisture control: warm indoor air leaks into the attic and carries water vapor. Moving air removes that vapor before it condenses on cold sheathing and framing.
• Heat reduction: in summer a roof deck can get very hot. Airflow under the deck lowers the temperature of the attic and the load on the ceiling below.
• Winter roof care: a cold attic roof helps limit uneven snow melt, which can lead to ice dams at the eaves.
• Material life: sheathing, framing and some shingle products last longer when the attic stays dry.

The key measure is net free area, or NFA. It is the open area of a vent that air can actually pass through, after the louvers, screens and baffles are subtracted. A vent with a large visible opening can have a much smaller NFA.

The calculator converts the attic floor area into a required net free area, splits it between intake and exhaust, and then divides each share by the NFA of one vent unit.

Step by step:

1. The attic floor area is divided by the ratio, 150 or 300, to give the required NFA in square feet.
2. That figure is multiplied by 144 to convert square feet to square inches, because vent labels list NFA in square inches.
3. The total is split into intake and exhaust. At 1/150 the calculator uses 60 percent for intake and 40 percent for exhaust. At 1/300 it uses an even 50/50 split.
4. The exhaust share is divided by the NFA of one exhaust unit: 18 sq in per linear foot of ridge vent, 144 sq in per 12 by 12 in gable vent, or 50 sq in per roof louver.
5. The intake share is divided by the NFA of one intake unit: 9 sq in per linear foot of continuous soffit, or 65 sq in per 8 by 16 in soffit vent.
6. Each quantity is rounded up to a whole vent or a whole foot.

Assumptions: the per-unit NFA values are typical planning figures, not product data. Real products vary, so replace them with the number printed on your vent.

An attic with 1,500 sq ft of floor area, the 1/150 ratio, a ridge vent and continuous soffit vent:

1. Required NFA in square feet: 1,500 / 150 = 10.0 sq ft
2. Required NFA in square inches: 10.0 x 144 = 1,440 sq in
3. Intake share at 60 percent: 1,440 x 0.60 = 864 sq in
4. Exhaust share at 40 percent: 1,440 x 0.40 = 576 sq in
5. Ridge vent at 18 sq in per linear foot: 576 / 18 = 32 linear ft
6. Continuous soffit at 9 sq in per linear foot: 864 / 9 = 96 linear ft

The result is 1,440 sq in of total NFA, with 32 linear feet of ridge vent and 96 linear feet of continuous soffit vent. If the same attic used individual 8 by 16 in soffit vents at 65 sq in each, the intake count would be 864 / 65 = 13.3, which rounds up to 14 vents. With the balanced 1/300 ratio the total drops to 720 sq in, split 360 and 360, which needs 20 linear ft of ridge vent and 40 linear ft of continuous soffit.

Attic Floor Area

The area is the only size input in the formula, so the required NFA grows in direct proportion to it. Doubling the attic floor area doubles the vent area you need.

Ventilation Ratio

The 1/150 ratio asks for twice the vent area of the 1/300 ratio. The lower ratio is allowed only when specific conditions are met, such as a balanced system with part of the vent area high in the attic. Check the current local code before you rely on it.

Balance of Intake and Exhaust

Air only moves well when it can enter and leave. Too much exhaust and too little intake can pull conditioned air from the house or draw rain and snow in through exhaust vents. A rule of thumb is to give intake at least as much area as exhaust.

Vent Type and Product NFA

Two vents with the same outside size can have very different net free areas. Always use the NFA stated by the manufacturer for the product you buy.

Insulation and Baffles

Blown or batt insulation can block soffit vents. Baffles keep a clear channel from the soffit to the attic so the intake area can actually work.

Roof Shape and Climate

Hip roofs have short ridges, which limits how much ridge vent you can fit. Cold and humid climates raise the importance of moisture control, while hot climates focus more on heat relief.

No prices are stated here because vent, labor and roofing costs change by region and by season. For the costs of your roof, request written quotes. A ridge vent paired with a continuous soffit vent is the most common balanced system. Do not mix a ridge vent with gable vents on the same attic, because the gable vents can short-circuit the airflow.

A Balanced System

The 1/300 ratio fits when intake and exhaust are balanced and a significant part of the exhaust area sits high in the attic, near the ridge. A ridge vent with a continuous soffit vent is the classic example.

A Vapor Retarder on the Warm Side

Codes often allow the lower ratio when a vapor retarder is installed on the warm side of the ceiling. This limits the moisture that reaches the attic in the first place.

When to Stay with 1/150

If you cannot confirm either condition, use 1/150. It is the standard requirement and it gives more margin when vent locations are limited or the system is not balanced.

Check Local Rules

Local amendments and the code edition in force can change the allowed ratio. Confirm the rule with your building department before you finalize the design.

When a Different Approach Is Needed

Cathedral ceilings, spray foam on the roof deck and unvented roof assemblies follow different rules. This calculator is for conventional vented attics only.

1. Counting the Visible Opening Instead of the NFA

A vent that measures 12 by 12 in does not give 144 sq in of net free area once the louvers and screens are included. Use the figure from the product label.

2. Too Much Exhaust and Too Little Intake

Adding more ridge vent without more soffit vent will not help. The attic needs intake to match exhaust, or air will be pulled from the living space.

3. Mixing Vent Types

Combining a ridge vent with gable or louver vents can create short loops of air that never reach the lower attic. Use one exhaust type per attic.

4. Blocked Soffit Vents

Insulation pushed into the eaves blocks the intake path. Install baffles and check that the soffit vents are open to the attic.

5. Using the Roof Area Instead of the Attic Floor Area

The ratio applies to the attic floor area, not to the sloped roof surface. Measuring the roof will overstate the requirement.

The arithmetic is exact for the values you enter. The limits come from the simplified model, in particular the fixed per-unit NFA values and the fixed intake and exhaust split.

The standard rule is 1 square foot of net free vent area for every 150 square feet of attic floor. A 1,500 sq ft attic needs 10 sq ft, or 1,440 sq in, split between intake and exhaust.

Net free area, or NFA, is the open area of a vent that air can pass through after louvers, screens and baffles are subtracted. It is printed on the vent label, usually in square inches.

The lower ratio is generally allowed for a balanced system with part of the exhaust high in the attic, or when a vapor retarder is on the warm side of the ceiling. Check your local code edition first.

Many references suggest at least half of the vent area at the eaves for intake. This calculator uses 60 percent intake and 40 percent exhaust at 1/150, and an even split at 1/300.

A ridge vent moves air along the whole peak in any wind direction, so it is a common choice. Gable vents work in simple roofs but can leave dead zones, and should not be mixed with a ridge vent.

It is best not to. Gable vents can pull air across the attic and bypass the ridge vent, so the lower attic gets little airflow. Pick one exhaust type.

Yes, wherever insulation could cover the vent opening. Baffles keep a clear air channel from the soffit up into the attic.

Count the attic floor area. The ratio is defined against the floor area, not the sloped roof surface.

No. Those assemblies follow different rules and are often unvented. This tool is for a conventional vented attic only.

No. The calculator runs in your browser and nothing you enter is stored or sent anywhere.

• PNNL BASC Attic Ventilation (Passive ventilation calculation guide from the Building America Solution Center.)
• IRC Section R806 Roof Ventilation (Code requirements for attic ventilation ratios and balance.)
• GAF Ventilation Calculator (Manufacturer ventilation sizing tool and NFA reference.)
• Building Code Trainer – Roof Ventilation (Step-by-step guide to calculating attic NFA per IRC R806.)