Free Residential Ventilation Tool

Whole-House Fan & Attic Ventilation Fan Calculator

Two different fans, two different jobs. A whole-house fan pulls cool evening air through open windows and pushes hot air out through the attic — sized on the living space. An attic fan only exhausts the attic — sized on the attic floor. This tool sizes both and checks the often-forgotten part: enough vent area for the air to get out and enough window opening for it to get in.

No signup · Works on mobile · Formulas shown below · Updated 2026-08-23

Whole-House & Attic Fan Sizing

Whole-house: floor area × height × ACH ÷ 60 (10–20 ACH). Attic: 0.7 CFM per ft² (×1.15 for dark roofs). Net free area ≥ 1 ft² per 750 CFM.

Quick preset

What are you sizing?

Conditioned floor area

Area unit

Average ceiling height

ft / m

Air changes per hour (climate)

Attic floor area

ft² / m²

Roof colour / exposure

Existing attic vents (gable/soffit/ridge) net free area

ft²

Gable louvre 24×24 in ≈ 2 ft² gross, ~1.2 net. Ridge vent ≈ 18 in²/ft. Soffit strip ≈ 9 in²/ft.

Results

Engineering estimate for preliminary sizing. Verify against the fan curve, local codes and your system measurements before purchase. Ask a Longwell engineer to check your duty point free of charge.

Sizing rules

Whole-house fan CFM = floor area × ceiling height × ACH ÷ 60 // 10 ACH mild/quiet, 15 standard, 20 hot-dry
Quick rule CFM ≈ 2–3 × floor area (ft²)
Attic exit area net free area ≥ 1 ft² per 750 CFM // gable louvres, ridge vent, roof jacks
Window opening ≥ 1 ft² per 350 CFM, spread across rooms you want cooled

Attic fan CFM = attic floor area × 0.7 (× 1.15 dark roof / full sun)
Intake area ≥ 1 ft² per 300 CFM (soffit, low gable)
Passive only NFA = attic area ÷ 150 (÷ 300 with vapour retarder), half high, half low

Whole-house fan vs attic fan vs AC

	Whole-house fan	Powered attic fan	Central AC
Cools	Living space (and attic)	Attic only	Living space
Typical airflow	2,000–7,000 CFM	800–1,600 CFM	1,200–2,000 CFM
Power	200–700 W	50–300 W (or solar)	3,000–5,000 W
Works when	Outdoor air cooler & dry	Sunny days, hot attic	Always
Noise	Moderate; choose EC / insulated	Low (on roof)	Low indoors
Best climates	Hot-dry, Mediterranean, mountain	Hot with dark roofs	Humid

Frequently Asked Questions

What size whole house fan do I need?

Multiply the house volume (floor area × ceiling height) by 2–6 air changes per hour and divide by 60. A 2,000 ft² home with 8 ft ceilings (16,000 ft³) at 15 ACH needs 4,000 CFM; at 20 ACH 5,300 CFM. That matches the common rule of 2–3 CFM per ft² of floor area. Quiet, energy-saver installations use 10 ACH (≈ 2,700 CFM here) and run longer.

How much attic venting does a whole house fan need?

At least 1 ft² of net free area per 750 CFM of fan capacity — a 4,000 CFM fan needs 5.3 ft² of gable, ridge or roof vents. Gable louvre screens cut the gross opening roughly in half, so count net area.

What size attic fan for a 1,500 sq ft attic?

About 1,050 CFM (0.7 CFM/ft²), or 1,200 CFM for a dark roof in full sun. Provide at least 3.5–4 ft² of net intake area at the soffits.

Do attic fans actually help?

On hot days with a dark roof they cut attic temperature by 20–40 °F and reduce AC load modestly. If the attic floor is well insulated and air-sealed, the benefit is small; if it is leaky, a powered fan can pull conditioned air out of the house. Fix sealing and passive venting first, then add a fan if the attic still runs above 120 °F.

How many windows should be open with a whole house fan?

Enough to give about 1 ft² of opening per 350 CFM — for a 3,000 CFM fan, roughly 8–9 ft², e.g. four windows open 12 in × 24 in. Open windows in the rooms you want to cool and close the others so air flows through them.

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<a href="https://www.longwellfans.com/resources/whole-house-attic-fan-calculator/">Whole House Fan Sizing & Attic Fan Calculator</a> by LONGWELL Fans

APA: LONGWELL Fans. (2026). Whole House Fan Sizing & Attic Fan Calculator. https://www.longwellfans.com/resources/whole-house-attic-fan-calculator/

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