Estimate powered attic fan airflow, fan count, and intake net free area for a confirmed vented attic.
Table of contents
How to use our Attic Fan Size Calculator
- Choose Yes only if the attic is vented to outdoors and is outside your home's conditioned space.
- Enter the horizontal Attic floor area in sq ft, using length times width or a house plan.
- Choose Yes for Dark shingles or steep roof? if either condition applies.
- Optionally enter Airflow per fan you are considering and the combined Available outdoor-air intake NFA from vent specifications.
- Click Calculate and check that Outdoor-air intake check says Enough; a negative intake margin means more intake area is needed.
Definitions
CFM: Cubic feet per minute, a measure of how much air a fan moves.
Net free area (NFA): The open area air can actually pass through a vent, usually listed by the manufacturer in sq in. It is smaller than the vent's visible face size.
Conditioned space: The part of a home intentionally heated or cooled. A standard vented attic is normally outside this space.
Planned installed fan airflow: The combined rated CFM of the whole number of selected fans. It can exceed the recommended minimum.
Intake margin: Available intake NFA minus the minimum required intake NFA. A negative margin is a shortage.
Common mistakes and quick fixes
Mistake: Entering roof surface area or the entire home's area as Attic floor area.
Fix: Enter the horizontal floor area inside the attic in sq ft.
Mistake: Choosing Yes for Is this a vented attic outside your home's conditioned space? when insulation is at the roof deck.
Fix: Choose No or Not sure and confirm the attic design before using a powered-fan size.
Mistake: Entering the outside grille dimensions as Available outdoor-air intake NFA.
Fix: Add the manufacturer-listed net free area for all qualifying intake vents.
Mistake: Treating Recommended minimum fan airflow as the intake requirement after selecting a fan.
Fix: Use Minimum outdoor-air intake NFA, which accounts for whole-fan rounding when fan airflow is entered.
Mistake: Reading a negative Available intake above or below the minimum as zero.
Fix: Keep the minus sign: it is the sq in of intake NFA still needed.
Limitations & Key Assumptions / Boundary Conditions
- This is a planning-level quick method for a confirmed standard vented attic outside the conditioned space. It does not size a whole-house fan or a fan for an unvented, sealed, or roof-deck-insulated attic.
- The calculation uses 0.7 CFM per sq ft and adds 15 percent for dark shingles, a steep roof, or both. It does not model attic volume, local climate, duct losses, static pressure, humidity, or thermostat settings.
- Intake NFA must be the manufacturer-listed open area for outdoor-air intake vents serving the attic. Screens, baffles, blockage, and the vent layout can affect real airflow.
- The intake requirement is based on rated fan capacity. Product instructions, local building rules, roof design, electrical requirements, and installation conditions can require a different design.
- A positive intake margin only means the entered NFA meets this planning minimum. It does not confirm that a fan is appropriate for the house or that installation will solve heat, moisture, or comfort problems.
Methodology
Quick sizing calculation
For a confirmed vented attic, the calculator uses Home Ventilating Institute planning guidance: 0.7 CFM for each sq ft of attic floor area, with a 15 percent increase for dark shingles, a steep roof, or both. [1]
raw airflow (CFM) = attic floor area (sq ft) x 0.7 x roof factor
The roof factor is 1.15 when the roof adjustment is Yes; otherwise it is 1. The calculator rounds the result upward so the displayed target is not below the calculated airflow.
recommended minimum fan airflow = round raw airflow up to a whole CFM
Fan count and intake area
If airflow per fan is entered, the calculator rounds the fan count up to a whole fan. Intake sizing then uses the full installed fan capacity, rather than the lower airflow target.
fans needed = round recommended minimum fan airflow / airflow per fan up to a whole fan
required intake NFA (sq in) = planned fan capacity (CFM) / 300 x 144
The 300 CFM ratio means one sq ft of inlet NFA per 300 CFM of fan capacity. Multiplying by 144 converts sq ft to sq in. [1]
When available intake is entered, the calculator compares it with the required intake area. A negative margin is kept as a negative number because it shows the shortage.
intake margin (sq in) = available intake NFA - required intake NFA
Mini-example
A 1,000 sq ft attic with no roof adjustment has a raw target of 700 CFM, so the recommended minimum is 700 CFM. With 600-CFM fans, two fans are needed, producing 1,200 CFM of planned airflow. Required intake NFA is 1,200 / 300 x 144 = 576 sq in. If available intake NFA is 500 sq in, the margin is -76 sq in and the intake check is Not enough.