Exhaust Fan CFM Calculator (Bathroom, Whole House & Duct Fans)

What is CFM and why it matters

CFM (cubic feet per minute) is the amount of air an exhaust fan moves in one minute. In metric markets — the Middle East, Europe and Australia — the same figure is usually quoted as m³/h (cubic metres per hour). Both measure the same thing: how fast the fan can replace the air in a room.

The single biggest reason an exhaust fan "doesn't work" is that it was sized for the room's floor area instead of its volume and the air changes per hour (ACH) the application requires. A 50 CFM fan that is adequate for a small bathroom will do almost nothing in a 500 m³ restaurant kitchen.

The formula

For metric: m³/h = Room volume (m³) × ACH. To convert, 1 CFM ≈ 1.699 m³/h.

Air changes per hour (ACH) by application

Worked example: bathroom

A bathroom measuring 10 ft × 8 ft × 8 ft (volume 640 ft³) needs 8 air changes per hour:

You would select a fan rated at least 90–100 CFM after adding a small margin for the duct run.

Bathroom CFM quick-reference chart

The chart applies the formula at 8 ACH with a standard 2.4 m (8 ft) ceiling, so you can skip the maths for the most common room sizes:

For ducted installations — where the fan is not mounted in the wall itself — pick the model whose airflow still clears the target after duct losses, or step up to a mixed-flow inline fan, whose pressure curve holds up better under resistance.

Two things the calculator can't see

• Duct resistance (static pressure): every bend, grille and long run reduces real-world airflow. Add 10–20% to the calculated CFM, or use a fan with a steeper pressure curve (an inline or mixed-flow fan rather than a basic axial fan).
• Filters and shutters: backdraft shutters and insect screens reduce airflow. Size up rather than down — and pick the shutter type deliberately; see our exhaust shutter types guide.

How to calculate the CFM of an axial fan

An axial fan is rated at a specific test pressure, and the number on the datasheet is measured at a set static pressure — often 0 Pa, called free air. The real CFM you get depends on the resistance of your installation:

• Free-air rating: a fan advertising 984 CFM may deliver that only with no duct, no shutter and no grille attached.
• Operating point: real airflow is where the fan's pressure curve crosses your system's resistance curve — typically 20–40% below the free-air number for a wall installation with a shutter.
• Practical rule: if you must have 500 CFM at the blade, spec a fan whose free-air rating is 650–700 CFM, or choose a mixed-flow inline fan whose curve holds up under pressure.

This is why our product pages list CFM at rated pressure, not just free air — see the exhaust fan range for tested values per model.

Whole house fan size calculator

For whole-house ventilation the goal is 15–20 air changes per hour in hot climates (a shortcut rule is 30 CFM per 100 sq ft of floor area). Calculate the total volume of the conditioned space, then apply the same formula with a higher ACH:

Example: a 2,000 sq ft house with 9 ft ceilings has a volume of 18,000 ft³, so it needs 4,500 CFM of continuous extract. In practice this is delivered by several fans zoned by area — bathrooms and kitchens on their own controls — rather than one oversized unit. For hot, humid Gulf climates, run the fans continuously at low speed; see our Middle East climate ventilation guide.

Wall vs ceiling fan: does the CFM change?

The required CFM stays the same — air changes depend on room volume, not where the fan sits. What changes is how much of the rated CFM you actually get:

• Wall / window fans exhaust straight outside with almost no duct loss, so you get close to the rated CFM. They are the first choice for kitchens, workshops and ground-floor rooms — see the full comparison in our wall vs ceiling exhaust fan guide.
• Ceiling fans are ducted through the roof space, adding bends and resistance. Add 15–20% to your CFM target, or step up one model size.

Verify real airflow after installation

For commercial projects, consultants increasingly ask for ventilation verification — proof that the installed system delivers the design airflow. A simple commissioning check anyone can run:

1. Measure air velocity at the grille with an anemometer, averaged over a 5-point grid.
2. CFM = average velocity (fpm) × free grille area (sq ft).
3. Accept values within ±10% of the calculated design. If measured airflow is lower, check shutter drag, duct leaks and backdraft flaps first.

BAER can supply factory test curves and test reports for any model on request — useful for consultant-submitted projects and tender documentation.

Real orders sized with this method

The calculator on this page is not theoretical — it is the same sizing logic our export team runs when a distributor sends a room list. Four orders from our own shipping records:

The pattern matches everything above: long duct runs in malls and kitchens ended up on mixed-flow inline fans, while direct wall or window openings took axial units. Quantities come from our own order records; buyer names stay confidential.

Frequently asked questions

Related sizing guides

This calculator gives you the airflow target. These guides cover what a calculator cannot see — static pressure, duct resistance and application-specific margins:

• How to calculate exhaust fan airflow (CFM & m³/h) — the air-changes-per-hour formula step by step, unit conversion, and a worked Gulf-region example.
• Bathroom exhaust fan CFM: how much do you need? — CFM by bathroom size plus the duct margin most buyers forget.
• Panel exhaust fan sizing guide — sizing the flat panel fans used in wall and window openings.
• Inline duct fans: sizing and selection — when a ducted inline fan beats a wall or ceiling fan.