Whole House Fan Size Calculator
Whole house fan CFM, vent, and window area.
What it does
Whole house fans cool with windows instead of compressors, but only when sized to the house. A 2,000 sq ft home needs 6,000 CFM at the full-breeze rule with 8 sq ft of attic vent and 24 sq ft of open windows. Undersized fans stir disappointment; starved attics pressurize trouble.
The tool above runs HVI sizing with DOE cross-checks. Enter floor area, ceiling height, and breeze level, and it prints CFM, attic net-free area, and window area with ACH in the note. Defaults give 6,000 at 22.5 ACH; moderate gives 4,000 at 15.
Both authorities agree. HVI tables sqft-times-3 with CFM-over-750 venting, and DOE Building America confirms 1.5-3 CFM per sq ft with 12-23 ACH, 1 sq ft of vent per 750 CFM, and 4 sq ft of windows per 1,000 CFM. A published 1,500 sq ft case lands 4,500 CFM exactly.
Attic vents gate the whole system. Screens halve free area, so double calculated venting with screened louvers, and open windows on cool sides before switching on. Run evenings and nights; close up against hot dusty afternoons.
How to use
- Enter the home floor area.
- Enter the ceiling height.
- Pick full or moderate rate.
- Read CFM first, then attic vent.
- Read open window area.
- Run on cool evenings.
Formula
- sq ft
- Home floor area in sq ft.
- height
- Ceiling height for the ACH check.
- rate
- 3 full breeze, 2 moderate.
HVI sets x3 with CFM/750 venting; DOE BASC confirms 1.5-3 CFM/sq ft with 12-23 ACH and 4 sq ft windows per 1,000 CFM.
Worked example
Example: 2,000 sq ft home
Defaults: 2,000 sq ft with 8 ft ceilings, full breeze.
- Inputs: 2,000 sq ft, 8 ft ceilings, full 3 CFM/sq ft.
- CFM: 2,000 x 3 = 6,000.
- Attic vent: 6,000 / 750 = 8 sq ft.
- Windows: 6,000 / 250 = 24 sq ft open.
- ACH: 6,000 x 60 / 16,000 = 22.5. Tool shows 6,000 CFM.
Reference table
| Home | CFM | Vent / Windows |
|---|---|---|
| 1,000 sq ft | 3,000 | 4.0 / 12 |
| 1,500 sq ft | 4,500 | 6.0 / 18 |
| 2,000 sq ft | 6,000 | 8.0 / 24 |
| 3,000 sq ft | 9,000 | 12.0 / 36 |
Limits
- Planning math only. Ducts and doors shift flow.
- Screens halve free vent area.
- Climate decides usable hours.
- Sealed attics need pro design.
Common mistakes
- Sizing CFM without attic venting.
- Running with windows closed.
- Skipping screen halving on vents.
- Daytime use in hot dusty air.
- Expecting AC dehumidification.
Try it yourself
Quick checks. Answers sit tucked under each question.
2,000 sq ft moderate rate?
CFM 2,000 x 2 = 4,000. Vents 4,000/750 = 5.3 sq ft. Windows 4,000/250 = 16 sq ft.
1,500 sq ft, 9 ft ceilings, full?
CFM 1,500 x 3 = 4,500. Vents 4,500/750 = 6 sq ft. Windows 4,500/250 = 18 sq ft.
Questions
How is fan CFM figured?
Floor area times rate: 2,000 x 3 = 6,000 CFM full breeze; x2 gives 4,000 moderate.
How much attic vent?
CFM over 750: 6,000/750 = 8 sq ft net free area (double with screens).
How much window area?
CFM over 250: 6,000/250 = 24 sq ft of open windows.
What ACH results?
CFM x 60 over volume: 22.5 here against the 12-23 effective band.
When should it run?
When outside air beats inside, evenings and nights. Close up for hot dusty days.
Do hot climates differ?
Yes. Hot climates size up; the x0.4 option pairs small fans with paddle fans.
Sources
- HVI: How Much Ventilation (sqft x3, CFM/750)
- HVI: Whole-House Comfort Ventilators
- DOE Building America: Whole-House Fans