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Attic Ventilation Calculator

Estimate net free ventilation area and equal intake/exhaust vent counts from attic floor area and a selected ventilation ratio.

sq ft
Enter a number from 1 to 1000000.
Choose a listed value.
%
Enter a number from 1 to 99.
sq in
Enter a number from 1 to 10000.

Use the units shown. Blanks, words, comma-formatted numbers, fractions, non-finite values and values outside the stated ranges are rejected.

Total net free area
Minimum intake vents
Minimum exhaust vents

How to use this calculator

  1. Replace every example value with your own measurements, label values or planning assumptions.
  2. Keep each value in the unit printed beside its field.
  3. Select Calculate, then compare the result with the formula and limitations below.

Formula and method

Total net free area in square inches = attic floor square feet ÷ selected ratio × 144. Intake and exhaust shares are divided by the labeled net free area per vent and rounded up.

Results are hidden whenever an input changes or validation fails, so an older answer cannot be mistaken for the current calculation.

Worked example

Attic floor area = 1500 sq ft; Ventilation ratio = 300; Intake share = 50 %; Net free area per vent = 50 sq in. The calculator returns Total net free area = 720 sq in; Minimum intake vents = 8 vents; Minimum exhaust vents = 8 vents.

The worked example uses the same default values displayed in the calculator.

Limitations and safety

Ventilation ratios and vapor-barrier exceptions depend on code, climate and roof design. Use manufacturer net-free-area ratings, avoid blocked intake, and confirm the design with the local building authority.

Frequently asked questions

What should I enter first?

Start with measurements or rates from the project, product label, service quote or equipment specification. Replace every example value before relying on the estimate.

Why can the real result differ?

Ventilation ratios and vapor-barrier exceptions depend on code, climate and roof design. Use manufacturer net-free-area ratings, avoid blocked intake, and confirm the design with the local building authority.

Does the calculator round purchases up?

Only outputs labeled as whole packages, vents, heaters, pails or similar counts are rounded up. Keep extra precision for planning and follow supplier packaging.

Review notes and reference

Reviewed October 5, 2026. The formula was independently checked against the displayed default example, and the calculator was tested for valid defaults, empty input, invalid formats, range boundaries and recovery. Reference: NIST SI Units. Explanations and examples are original.

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