Sizing and ventilation
Attic ventilation calculator
Enter attic floor area for vent area, intake/exhaust split and vent counts.
Size your attic vent area
Example inputs · edit for your homeYour ventilation result
Enter your measurements to see the airflow or vent area.
Methods: HVI ventilation guide and 2021 IRC R806.2 training summary (PDF pages 61-62).
What's covered7
This attic ventilation calculator plans passive vent net free area. Enter the attic footprint and select the ratio. Identify the roof assembly too. It does not calculate electric fan airflow. Its result is arithmetic for a vented space.
The result does not prove a local-code exception applies. It also cannot show that every rafter channel is open.
Use the 1:150 baseline when the code exception is unknown. A 1:300 comparison has conditions. Choose the low-intake and high-exhaust split. Use the exact vent's rated free area for a count. Without that rating, the count stays unknown. Check an unvented or unidentified roof design before adding vents.
The Size tab accepts square feet or square metres and converts measurements when units change. Vent-area and product inputs keep their labelled square-inch units. If existing vent area is known, the product tab shows both the full requirement and the additional vents or length needed. Copy the result or a link to the entered plan, or print the worksheet.
Attic ventilation calculator method and units
Net free area is the open part of a vent after its screen and louvers are accounted for. It differs from the face dimensions. Use the vent maker's rating. Do not apply an extra guessed open-area factor to a rating that already describes net free area.
The total planning area equals attic floor area divided by the selected denominator. At 1:150, divide by 150. Multiply square feet by 144 to get square inches. The HVI static-ventilation guide describes that unit conversion and a low/high allocation method.
Use the attic footprint, not total floor area from all stories of the house. A two-story home can have a much smaller attic floor than its total living area. Do not include a separate garage attic unless it is part of the same assessed vented space. Record separate spaces separately.
1:150 baseline and conditional 1:300
The ICC-hosted 2021 IRC plan-review slides (PDF pages 61-62) sets a 1:150 baseline and a conditional 1:300 exception. That named model-code edition is not a statement of what every jurisdiction has adopted. Check the applicable edition and amendments for the actual project.
The exception has rules for vent locations. In the named cold zones, it also has a ceiling vapor-retarder rule. A total vent-area number alone cannot confirm those conditions. Record the edition and jurisdiction if you mark the exception verified. That is your record, not the tool's inspection.
When you choose 1:300 with unknown exception status, the operative result stays at the 1:150 baseline. The smaller total is shown only as an illustrative comparison. This keeps a convenience choice from being displayed as a code finding.
Attic ventilation calculator intake and exhaust allocation
HVI's static-vent guidance uses 60% low intake and 40% high exhaust. That is the default planning split in the form. You can enter another split that sums to the whole total. The split is a planning input, not a substitute for the location rules in an applicable code.
Low intake belongs to a clear path from outdoors into the roof space. High exhaust lets attic air leave. A pair of well-rated vents can still fail to connect if insulation blocks the path. Confirm the eave channel and any divisions in the roof. See the attic ventilation guide for that assembly check.
Worked arithmetic and a manual worksheet
For an illustrative 1500-square-foot vented attic, the baseline calculation is 1500 / 150 = 10 square feet. Multiply by 144 for 1440 square inches total. At the default split, low intake is 864 square inches and high exhaust is 576 square inches. These are example inputs, not measurements of your attic.
If a hypothetical intake vent provides 50 square inches per unit, 864 ÷ 50 = 17.28. Round up to 18 whole vents. The example rating is invented for arithmetic only; it does not describe a named product. A real count needs an identified product and its documented rating.
| Manual worksheet item | Your entry or calculation |
|---|---|
| Vented attic floor area | Measured square feet |
| Chosen denominator | 150 baseline, or a verified conditional 300 |
| Total net free area | Floor area / denominator |
| Total square inches | Total square feet × 144 |
| Low intake requirement | Total square inches × chosen intake fraction |
| High exhaust requirement | Total square inches × chosen exhaust fraction |
| Whole vent count | Round required side area / per-vent NFA upward |
| Linear vent length | Required side area / NFA per linear foot |
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The tool's product count covers the full selected side. It does not subtract existing vents from that count. Existing area is checked separately for a shortfall. These are two different tasks. Keep the old-vent inventory separate from the full new-product count.
Sources, limits and next steps
The methods were checked against HVI guidance and the ICC-hosted training summary on October 15, 2026. The PDF is a training summary of the named edition, not the adopted local code. For requirements and exception details, read our requirements guide. The source edition and adopted local rules should travel with the project record.
Compare component roles after the assembly and planning area are clear. See the attic ventilation guide.
Compare the published ventilation and whole-house cooling figures when checking which air path a number describes.
FAQ
Does this size a powered attic fan?
No. This calculates passive vent area. A powered attic fan has a model-specific flow and intake requirement. The whole-house cooling outlet calculation is different too. Do not apply this passive ratio to a motor selection without checking the exact equipment and the intended air path.
Should I count the whole vent grille?
Use the documented net free area, not the grille's outside dimensions. Screens and louvers reduce the open portion. Blocked passages behind the vent also need inspection. A correct label does not prove that insulation leaves a clear route into the attic.
Why is no vent count displayed?
The count needs a positive per-unit rating and an identified product. If either is missing, the tool leaves the count unresolved. It can still calculate total area for a known vented assembly. Check the current product sheet before entering the missing rating.
Can an unvented attic use the result?
The tool withholds a prescriptive result for an unvented or unknown assembly. Adding openings can work against a sealed roof design. Identify the insulation and air boundary first. Follow the assembly requirements rather than turning an unknown roof into a vented one based on arithmetic.
Verdict
Use the attic ventilation calculator for a known vented assembly. State the ratio and product rating. Confirm the local exception first. Check the real air path too. A vent count needs those facts before it becomes an installation plan.