How this calculator works
A power attic fan needs about 0.7 cubic feet of air per minute for every square foot of attic floor. For a dark roof or a steep one the attic gets hotter, so the figure is raised 15% to 0.805. The fan has to be able to pull that air in from somewhere, so the HVI rule is one square foot of soffit intake (net free area) for every 300 CFM.
Worked example: a 50 × 30 ft attic is 1,500 ft², so it needs 1,050 CFM with a standard roof and 1,208 CFM with a dark one. Intake for the standard case is 1,050 ÷ 300 = 3.50 ft², or 504 in² of net free soffit vent area. A 2,500 ft² attic needs 1,750 CFM.
Attic fan size by attic floor area
CFM at 0.7 per square foot (standard roof) and 0.805 per square foot (dark or steep roof), with the soffit intake needed.
| Attic floor | Standard roof | Dark or steep roof | Intake (standard roof) |
|---|---|---|---|
| 800 ft² | 560 CFM | 644 CFM | 269 in² |
| 1,000 ft² | 700 CFM | 805 CFM | 336 in² |
| 1,200 ft² | 840 CFM | 966 CFM | 403 in² |
| 1,500 ft² | 1,050 CFM | 1,208 CFM | 504 in² |
| 1,800 ft² | 1,260 CFM | 1,449 CFM | 605 in² |
| 2,000 ft² | 1,400 CFM | 1,610 CFM | 672 in² |
| 2,500 ft² | 1,750 CFM | 2,012 CFM | 840 in² |
Tips before you buy
- Check the intake first. Add soffit vents or clear blocked ones before you buy a fan: the fan is only as good as the air that can flow in.
- A thermostat switch turns the fan on at about 100 to 110 degrees F so it does not run for nothing. A humidistat is useful in winter.
- Solar attic fans are rated in CFM too, and are usually smaller (about 800 to 1,600 CFM). Match them to the same sizing rule, or use two.
- If there is a gas furnace or water heater in the house, a strong attic fan can pull exhaust gas down its flue (backdrafting). Have it checked, and think twice if the house is very airtight.
- Air sealing and attic insulation matter as much as a fan: a hot attic costs you through the ceiling.
Tools & materials for this project
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Frequently asked questions
What size attic fan do I need?
Multiply the attic floor area by 0.7 to get the CFM, and by 0.805 for a dark or steep roof. A 1,500 ft² attic needs 1,050 CFM, or 1,208 CFM with dark shingles.
How many CFM per square foot of attic?
About 0.7 CFM per square foot of attic floor, raised by 15% to 0.805 for a dark or steep roof.
How much intake does an attic fan need?
One square foot of net free soffit vent area for every 300 CFM of fan. A 1,050 CFM fan needs 504 in² (3.50 ft²).
Can an attic fan be too big?
Yes. A fan that is too big for the intake pulls air from the house through ceiling gaps, wasting cooling, and can backdraft a gas furnace or water heater. Size the fan and the intake together.
Do I need an attic fan if I have a ridge vent?
Usually not, and mixing the two is a mistake: the fan can pull air in through the ridge vent instead of the soffit. A balanced soffit and ridge system, plus insulation and air sealing, works without a fan.
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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.