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The total floor area of the attic space.
Measure the attic's footprint the same way you would a room — length x width, adding sections for irregular shapes.
Minimum fan CFM
756 CFM (minimum)
Dark roofs, steep pitches, and hot climates all increase attic heat load beyond this baseline rule of thumb — size toward the higher end of available fan models if any of those apply.
- Attic area
- 1,080 sq ft
- Same airflow in L/s
- 356.79 L/s
They open the calculator with your figures already in it
Attic Fan CFM Calculator: 756 CFM (minimum) — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Home Ventilating Institute powered attic ventilator sizing: 0.7 CFM per square foot of attic floor area
Inputs used
- Attic Floor Area
- 1080 sq ft
Intermediate steps
- Attic area
- 1,080 sq ft
- Same airflow in L/s
- 356.79 L/s
Confidence note: Dark roofs, steep pitches, and hot climates all increase attic heat load beyond this baseline rule of thumb — size toward the higher end of available fan models if any of those apply.
What this calculation does not cover
- Floor area is the only geometry this uses. Roof pitch, ridge height and attic depth never enter the formula, so a shallow hip and a steep gable over the same footprint return the same CFM even though the steep one holds far more air to turn over.
- It does not check that the attic has enough intake. A fan sized from this number still needs matching net free intake area at the soffits; starved of intake it pulls make-up air out of the house through ceiling leaks, which can depressurise the top floor and backdraft an atmospherically vented water heater or furnace. Size the intake separately before you buy the fan.
- Nothing adjusts for roof colour, climate, insulation level, radiant barrier, or existing ridge and gable venting — the same divisor is applied to every attic. Dark roofs, steep pitches and hot climates all put the real heat load above this baseline.
- This is a rule-of-thumb equipment sizing figure, not a code calculation. Codes govern attic ventilation as net free vent area against attic floor area, and fitting a powered fan does not remove that requirement.
- Rated CFM is not delivered CFM. Shutters, insect screens, a clogged louver and static pressure at the opening all cut what the fan actually moves, and the figure here makes no allowance for that, for the thermostat or humidistat setpoint, or for splitting the airflow across more than one fan.
Add the equipment this sizes
This result is a specification — 756 CFM (minimum) — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Home Ventilating Institute powered attic ventilator sizing: 0.7 CFM per square foot of attic floor area
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