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Attic Ventilation Calculator

Estimate recommended attic ventilation area from attic square footage and a common ventilation ratio.

Updated April 13, 2026
Free online tool
Planning and research use

Why this page exists

Attic ventilation planning is easier when attic size is translated into a simple net free ventilation target instead of guessed from rules half remembered. This calculator helps visitors estimate recommended ventilation area and a simple intake-and-exhaust split from attic square footage.

Interactive tool

Run the estimate

Enter your numbers and read the result first, then use the sections below to understand what affects the outcome.

Calculator

Attic ventilation calculator

Estimate recommended attic ventilation area from attic size and a common ventilation-ratio assumption.

Result

3.29 sq ft NFA

Estimated recommended net free ventilation area based on attic square footage divided by the ventilation-ratio assumption selected.

Total attic area988.0 sq ft

Recommended net free ventilation area3.29 sq ft

Recommended NFA in square inches474.2 sq in

Suggested intake / exhaust split1.65 sq ft intake and 1.65 sq ft exhaust

38.0 by 26.0 feet gives about 988.0 square feet of attic area.
Using the 1:300 rule of thumb, that points to about 3.29 square feet of net free ventilation area.
Use the result as a planning estimate only, because intake and exhaust balance, insulation details, and local code can change the actual requirement.

This is a rule-of-thumb planning estimate, not a code decision. Local building-code requirements and roof-assembly details can change the net free ventilation area actually needed.

Planning note

Last updated April 13, 2026. Use this tool to compare scenarios and plan ahead, then confirm important details with the lender, employer, insurer, contractor, or other qualified provider involved in the final decision.

How it works

What the calculator is doing

Enter attic length and width.

Choose a ventilation ratio such as 1:150 or 1:300.

The calculator estimates total attic area, recommended net free ventilation area, and a simple intake-and-exhaust split.

Understanding your result

This is a rule-of-thumb estimate, not a code decision. Local requirements and roof-assembly details can change the actual ventilation target.

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Examples

Ways people use this tool

Example scenarios help turn a quick estimate into a more useful comparison or planning step.

Turn attic size into a ventilation target

A simple net free area estimate can help frame early planning before product selection starts.

Compare 1:150 and 1:300 assumptions

Switching the ratio can show how sensitive the recommendation is to the rule being used.

Use it with other attic and comfort tools

Ventilation estimates often fit naturally alongside insulation, BTU, and comfort-planning tools.

FAQ

Common questions

How is ventilation area estimated here?

The calculator divides total attic area by the selected ventilation ratio to estimate recommended net free ventilation area.

Why show intake and exhaust split?

A simple half-and-half split gives a practical starting point for thinking about balanced intake and exhaust ventilation.

Why is this not a code answer?

Local code, insulation details, vapor control, and roof assembly conditions can all change the actual ventilation requirement.

Related tools

Keep comparing

Use these related tools to compare nearby scenarios, check a second estimate, or keep narrowing down the right decision.

Home ToolsUpdated April 12, 2026