Soffit Vent Air Flow Calculator

Estimate attic intake net free area, soffit vent linear feet, slot free-area ratio, pressure-driven airflow, and intake-to-exhaust balance from one focused worksheet.

1.Start with a real soffit scenario

Presets set attic area, vent type, pressure differential, blockage, and exhaust NFA. You can adjust every value afterward.

2.Enter attic and vent details

Attic floor area (sq ft) Use the insulated attic floor footprint, not roof surface area.

Ventilation rule Total required NFA equals attic area divided by the selected rule.

Intake share of total NFA (%) Many balanced designs use 50 to 60 percent intake.

Soffit vent airflow/spec type Select rated NFA per foot, or choose custom to model slot area.

Rated NFA per linear foot (sq in/ft) For plug vents, convert quantity to equivalent feet or use custom NFA.

Available soffit vent length (ft) Total vented length across both eaves, after end gaps.

Pressure differential (Pa) Stack and wind effects often sit near 1 to 6 Pa.

Installed exhaust NFA (sq in) Include ridge, roof, or gable exhaust net free area.

3.What the calculator checks

NFA rule sizing Total attic NFA is attic floor area divided by 150, 240, or 300, then split into intake and exhaust targets.

Vent slot area model Custom slot NFA uses slot width x 12 x free-area ratio per foot, then applies blockage derate.

Pressure airflow model CFM is based on Q = Cd x area x square root of 2 x delta P / air density, then converted to cubic feet per minute.

Intake/exhaust balance The balance score compares effective intake NFA with exhaust NFA and flags starved, balanced, or intake-heavy attic airflow.

Balanced intake plan

Ready

Required intake NFA 288 sq in target

Required vent length 37.7 linear ft after derate

Estimated intake airflow 280 CFM through effective NFA

Intake/exhaust balance 100% intake vs exhaust NFA

Calculation breakdown

Attic floor area and ventilation rule1200 sq ft at 1/300

Total required attic NFA576 sq in

Target intake and exhaust split288 sq in intake / 288 sq in exhaust

Vent NFA per foot before blockage9.0 sq in/ft

Effective NFA per foot after derate7.65 sq in/ft

Installed effective intake NFA306 sq in

Pressure differential airflow formulaCd 0.62, delta P 3 Pa

Balance recommendationIntake and exhaust are closely matched.

4.Soffit vent airflow/spec grid

5.5 Perforated panel NFA

Lower flow per foot, useful when vented panels span long eave runs.

9-11 Standard strip NFA

Common continuous strip vents balance moderate attic exhaust systems.

18 High-flow strip NFA

Higher rated vents help when eave length is limited by returns or dormers.

0.62 Typical discharge Cd

Screened slot airflow is usually below ideal open-area flow.

5.Reference tables

Vent type	Typical NFA	Free-area ratio clue	Best use
Perforated soffit panel	4 to 7 sq in/ft	25 to 40 percent	Long eaves where the whole soffit run can breathe.
Vinyl continuous strip	8 to 10 sq in/ft	40 to 55 percent	Balanced 1/300 attics with typical ridge vent exhaust.
Aluminum strip with screen	9 to 12 sq in/ft	45 to 60 percent	Retrofits that need durable, even intake along both eaves.
High-flow continuous intake	16 to 20 sq in/ft	65 to 80 percent	Short eaves, large attics, or high exhaust NFA systems.

Attic floor area	Total NFA at 1/300	Target intake at 50%	Strip length at 9 sq in/ft with 15% derate
800 sq ft	384 sq in	192 sq in	25.1 linear ft
1200 sq ft	576 sq in	288 sq in	37.6 linear ft
1800 sq ft	864 sq in	432 sq in	56.5 linear ft
2400 sq ft	1152 sq in	576 sq in	75.3 linear ft

Pressure differential	What it represents	Flow effect	Calculator use
1 Pa	Light stack effect	Low natural flow	Use for mild weather or low-slope roofs.
3 Pa	Moderate attic draft	Baseline estimate	Good default for balanced natural ventilation checks.
6 Pa	Strong wind or thermal lift	About 1.4x the 3 Pa flow	Use for sensitivity, not guaranteed continuous airflow.
10 Pa	Very strong driving force	High but intermittent	Useful for stress checks on restrictive vent layouts.

Balance ratio	Condition	Likely issue	Planning target
Below 80%	Intake starved	Exhaust can pull from leaks or conditioned space.	Add effective soffit NFA or reduce exhaust area.
80 to 120%	Balanced	Good intake/exhaust pairing for most attic layouts.	Keep intake open with baffles and clean vent paths.
120 to 150%	Intake generous	Usually acceptable when exhaust is continuous and clear.	Confirm exhaust is not blocked at the ridge.
Above 150%	Exhaust limited	Extra intake may not raise real attic airflow.	Check exhaust NFA before adding more soffit vent.

6.Practical sizing notes

Use effective NFA for decisions The calculator separates rated vent NFA from actual effective intake after blockage. Painted screens, compressed insulation, missing baffles, and dust can turn a good-looking soffit run into a much smaller airflow path.

Balance before chasing CFM Natural attic airflow changes with wind and temperature, so the CFM number is a physics estimate. A closer intake/exhaust NFA balance is the more reliable design check for soffit vent planning.

Soffit vent are component that allow for outdoor air to enter the attic. Soffit vents are necessary because warm air create a rising movement that requires an escape route out of the attic through upper vent. If the soffit vents dont allow enough air to enter the attic, heat will build up in the attic.

This heat will make the house warmer. Furthermore, the attic will pull air from the conditioned room within the house. This will lead to increased cooling cost for the house.

How to Size and Check Soffit Vents

Therefore, you must ensure the soffit vents provide enough air to the attic to balance the exhaust vent that are within the attic. To calculate the needed size of the soffit vents, you must measure the attic roof to determine the square footage of the attic floor. The 1/300 rule is used in most instance to determine the needed size of the soffit vents.

The 1/300 rule states that one square inch of net free area is required for every 300 square feet of attic floor. Some attic may require more net free area if the pitch of the roof is steep or if the attic feature heavy insulation that may block the air from reaching the soffit vents. Furthermore, the intake air from the soffit vents must be balanced with the exhaust air from the ridge vents in the attic.

The net free area of the soffit vents is not always the same than the effective net free area of the soffit vents once installed into the attic. There may be a blockage factor due to the paint, screens, and insulation within the attic that might reduce the air movement through the soffit vents. For instance, the net free area of the soffit vents may be rated at nine square inches, but the effective net free area may be only seven square inches.

In this case, a calculator that takes into consideration the blockage factor of the soffit vents may be use to ensure that the soffit vents provide enough air to the attic. Airflow is created by the difference in air pressure in two area. These difference in air pressure can be created by the stack effect or by the movement of the wind.

The stack effect creates a movement of air within the attic due to the tendency of warm air to rise within the attic; warm air within the attic move out through the exhaust vents. A calculator can be used to calculate the cubic feet of air that moves through the attic each minute (CFM). This number can be used to determine whether or not the soffit vents will provide enough air to the attic.

However, the number calculated by the calculator is only an estimation of the airflow due to the changing speed of the wind. The actual amount of air that moves through the attic will change based off the movement of the wind. One of the common solution for increasing airflow through the attic vents is to increase the length of the vents.

However, this isnt always the best solution. If the exhaust vents have a larger diameter than the intake capacity of the soffit vents, the exhaust vents will create a pull on the attic air that will restrict the intake of air through the soffit vents. This will prevent the attic from effective venting its warm air.

Therefore, the intake area of the soffit vents should be balance with the exhaust area of the ridge or gable vents to ensure that the intake and exhaust vents are both effective. Finally, there may be physical obstruction to the movement of air within the attic. These physical obstruction might include recessed lights, return duct for the air conditioning system, or the insulation within the attic.

All of these can block the movement of air through the attic. A flashlight can be used to inspect the area where the soffit vents are located to ensure that there are no physical obstruction of the soffit vents. Even if the soffit vents have a large net free area, they will not function effective if the insulation within the attic blocks air movement to these vents.

By ensuring that the soffit vents have enough air movement, as well as ensuring that the exhaust vents effectively release the warm air from the attic, the attic will remain dry and cool. Furthermore, if the intake and exhaust vents are sized correctly, the attic will function proper and will not pull air from the conditioned room of the house. You should of checked the vents for any moddern obstruction that could stop the airflow.