Choosing an exhaust fan is not only about buying a 6-inch or 8-inch model. The fan must move enough air for the room, work against the resistance of the duct and remain quiet enough that people will actually use it.
For a bathroom or washroom, the main job is removing moisture and odour. For a kitchen connected to a dining area, the first choice should normally be a range hood that captures cooking fumes close to the stove and discharges them outdoors.
Quick Exhaust Fan Selection Table
| Application | Easy starting point | What else to check |
|---|---|---|
| Small bathroom, up to 50 ft² | Minimum 50 CFM | Duct length, bends, noise and outdoor discharge |
| Bathroom, 51–100 ft² | About 1 CFM per ft² | Select the next available fan size above the calculation |
| Bathroom above 100 ft² | Size by fixtures | 50 CFM each for toilet, shower and bathtub; 100 CFM for a jetted tub |
| Wall-mounted kitchen range hood | About 100 CFM per linear foot of hood width | Cooking load, duct size, makeup air and local code |
| Island kitchen range hood | About 150 CFM per linear foot of hood width | Island hoods need more airflow because they have no wall to assist capture |
| Enclosed kitchen/dining room general exhaust | Start with at least 5 air changes per hour | This does not replace a proper range hood over the cooking source |
Exhaust Fan Selection Helper
Use this as a practical starting point, then confirm the final selection using the manufacturer’s airflow data and your local requirements.
Your Starting Selection
Final check: Confirm that the selected fan can deliver the required airflow at the actual duct static pressure. Choose the next standard model size above the calculated figure rather than below it.
How to Size a Bathroom or Washroom Exhaust Fan
For bathrooms up to 100 square feet, the Home Ventilating Institute recommends approximately 1 CFM for each square foot of floor area. A 50 CFM fan is recommended as the minimum for bathrooms measuring 50 ft² or less.
For metric measurements, calculate the area in square metres and convert it to square feet:
Example: small bathroom
A bathroom measures 2.5 m × 2.2 m.
- Floor area = 5.5 m²
- Equivalent area = approximately 59.2 ft²
- Starting airflow = approximately 59 CFM
- Practical fan selection = 60, 70 or 80 CFM, depending on available models and duct resistance
Bathrooms larger than 100 ft²
For a large bathroom, HVI recommends adding the airflow requirement for each fixture:
| Fixture | Suggested airflow allowance |
|---|---|
| Toilet | 50 CFM |
| Shower | 50 CFM |
| Bathtub | 50 CFM |
| Jetted tub | 100 CFM |
A large bathroom containing one toilet, one shower and one bathtub therefore starts at 150 CFM. The room may use one properly located fan or more than one fan, depending on the layout.
How to Choose an Exhaust Fan for a Kitchen and Dining Area
Cooking pollutants should be removed close to the source. A ducted range hood is generally more effective than installing a wall exhaust fan far away in the dining area.
The US Department of Energy’s Building Science Education resource notes that kitchen exhaust should discharge directly outdoors rather than into an attic, crawlspace or space between floors.
Wall-mounted range hood
HVI’s recommended starting rate for a hood installed against a wall is approximately 100 CFM per linear foot of hood width.
A 30-inch hood is 2.5 feet wide:
- 2.5 × 100 = 250 CFM
- Starting selection = approximately 250 CFM
Island range hood
An island hood has no wall behind it to help contain the rising plume. HVI therefore recommends a higher starting rate of approximately 150 CFM per linear foot.
A 36-inch island hood is 3 feet wide:
- 3 × 150 = 450 CFM
- Starting selection = approximately 450 CFM
When a general kitchen/dining exhaust fan is used
A general wall or ceiling exhaust fan can help remove heat and background odour from an enclosed kitchen/dining space, but it is less effective at capturing grease and cooking fumes than a hood above the cooking appliance.
For a basic starting estimate, calculate room volume and use at least five air changes per hour unless the local code or project design requires another rate.
Understanding CFM and m³/h
Fan catalogues commonly use one of two airflow units:
- CFM: cubic feet per minute
- m³/h: cubic metres per hour
| CFM | Approximate m³/h |
|---|---|
| 50 CFM | 85 m³/h |
| 80 CFM | 136 m³/h |
| 100 CFM | 170 m³/h |
| 150 CFM | 255 m³/h |
| 250 CFM | 425 m³/h |
| 300 CFM | 510 m³/h |
| 450 CFM | 765 m³/h |
Why Fan Diameter Does Not Tell You the Airflow
Homeowners often ask whether they should buy a 6-inch, 8-inch or 10-inch exhaust fan. The opening diameter is useful for installation, but it does not prove how much air the fan will move.
Two fans with the same face size can have different:
- motor power;
- blade design;
- airflow ratings;
- noise levels;
- duct connections; and
- performance against static pressure.
Always check the certified airflow in CFM or m³/h. Use the diameter only after the airflow requirement is known.
Duct Length, Bends and Static Pressure
A fan may deliver its advertised airflow in a laboratory but move much less air after it is connected to a small, long or badly routed duct.
Resistance increases when the installation includes:
- long duct runs;
- several elbows;
- flexible duct that is compressed or sagging;
- a duct smaller than the fan outlet;
- dirty grilles;
- restrictive roof or wall caps; and
- backdraft dampers that do not open freely.
ENERGY STAR’s ventilating-fan criteria specifically evaluate installed performance at increased static pressure. This matters because a good fan should retain a useful portion of its rated airflow when connected to real ductwork.
- Keep the duct as short and straight as practical.
- Use the duct size recommended by the manufacturer.
- Avoid reducing the duct immediately after the fan.
- Use smooth rigid duct where possible.
- Stretch flexible duct fully and support it properly.
- Seal duct joints to prevent leakage.
- Discharge outdoors, not into the ceiling void.
How Quiet Should the Fan Be?
Bathroom and range-hood sound is commonly rated in sones. A lower number means quieter operation.
| Sound rating | How it may feel in a home |
|---|---|
| 0.3–1.0 sones | Very quiet; useful near bedrooms or for longer operating periods |
| 1.0–2.0 sones | Generally comfortable for normal household use |
| Above 2.0 sones | More noticeable; may discourage people from leaving the fan on long enough |
Current ENERGY STAR criteria set maximum sound levels of 2.0 sones for qualifying bathroom and utility fans from 10 to 200 CFM, and 2.0 sones for qualifying range hoods within the listed product category.
Wall, Ceiling and Inline Exhaust Fans
Wall fan
Simple when the bathroom or utility room has an external wall. A direct and short discharge path can reduce resistance.
Ceiling fan
Common in bathrooms where ductwork can run through the ceiling to an outside wall or roof termination.
Inline fan
Installed inside the duct, often above the ceiling. It can serve one or more grilles and may reduce room noise when designed correctly.
Axial or centrifugal?
An axial fan can work well for a very short, low-resistance path through an external wall. A centrifugal or mixed-flow fan is generally better when air must travel through a longer duct or several bends because it can handle more static pressure.
Timer, Humidity Sensor or Normal Switch?
| Control type | Best use | Possible limitation |
|---|---|---|
| Normal switch | Simple installations where the user reliably operates the fan | People may switch it off too soon |
| Run-on timer | Bathrooms and washrooms that need ventilation after use | Timer period must be set sensibly |
| Humidity sensor | Bathrooms with showers, condensation or irregular use | Poor adjustment can cause unnecessary operation |
| Occupancy sensor | Guest toilets and shared washrooms | Occupancy alone may not match the moisture-removal time |
| Variable-speed control | Kitchen hoods and applications with changing loads | Must be compatible with the fan motor and manufacturer’s instructions |
Worked Selection Examples
Example 1: bathroom with a short wall outlet
The bathroom measures 2.4 m × 2.0 m.
- Area = 4.8 m²
- Equivalent area = approximately 51.7 ft²
- Calculated airflow = approximately 52 CFM
- Practical selection = 60 or 70 CFM
- Recommended arrangement = quiet wall or ceiling fan with a timer
Example 2: bathroom with a long ceiling duct
The bathroom measures 3.0 m × 2.5 m, with approximately 8 m of duct and three bends.
- Area = 7.5 m²
- Equivalent area = approximately 80.7 ft²
- Base requirement = approximately 81 CFM
- Because the duct is restrictive, consider a fan around 100–110 CFM
- Confirm the airflow from the manufacturer’s fan curve at the expected static pressure
Example 3: wall-mounted kitchen hood
The hood is 900 mm wide and installed against a wall.
- 900 mm = approximately 3 ft
- 3 × 100 CFM = approximately 300 CFM
- Practical starting selection = 300 CFM or the next available model above it
- Use a smooth duct to the outdoors and check the appliance manufacturer’s requirement
Example 4: enclosed kitchen/dining room
The room measures 5 m × 4 m × 2.8 m and a general room exhaust calculation is required.
- Volume = 56 m³
- At 5 ACH: 56 × 5 = 280 m³/h
- Equivalent airflow = approximately 165 CFM
- This fan helps general air exchange but does not replace a range hood above the cooker
Common Exhaust Fan Buying Mistakes
- Choosing only by diameter: A large grille does not guarantee high airflow.
- Ignoring the duct: Long flexible duct can ruin the performance of a good fan.
- Buying exactly the calculated airflow: Select the next standard size above the requirement.
- Using a dining-room fan instead of a range hood: Cooking pollutants should be captured near the stove.
- Discharging into the attic: Moisture and grease must be exhausted outdoors.
- Ignoring noise: A fan that nobody uses is not an effective fan.
- No path for replacement air: Air cannot leave the room properly if no air can enter.
- Oversizing without checking makeup air: A powerful hood can depressurise a tight home.
- Installing a bathroom fan in a wet zone without the correct rating: Electrical location and product rating must follow local rules.
Installation and Safety Notes
- Use a qualified electrician where fixed wiring is required.
- Follow local electrical and mechanical codes.
- Use a fan suitable for the proposed bathroom zone and moisture exposure.
- Provide an adequate transfer-air path, such as a door undercut or transfer grille, where required.
- Keep the intake away from the main fresh-air opening so air sweeps across the room.
- Install a backdraft damper where appropriate.
- Keep the outlet away from openable windows and air intakes as required by local code.
- Make the fan and duct accessible for cleaning and maintenance.
- For gas cooking or fuel-burning appliances, check depressurisation and makeup-air requirements.
Homeowner Buying Checklist
| Item to check | What to look for |
|---|---|
| Airflow | CFM or m³/h at a stated test pressure |
| Installation type | Wall, ceiling, inline or range hood |
| Duct connection | Correct diameter and compatible outlet direction |
| Static-pressure performance | Fan curve or certified performance through ductwork |
| Noise | Sone rating at the relevant operating point |
| Control | Switch, timer, humidity sensor or variable speed |
| Backdraft protection | Built-in or separate damper |
| Energy use | Watts and airflow efficiency |
| Wet-area suitability | Manufacturer approval and local electrical requirements |
| Maintenance | Removable grille and accessible fan or filter |
Frequently Asked Questions
What size exhaust fan do I need for a small bathroom?
For a bathroom measuring 50 ft² or less, 50 CFM is a common minimum starting point. Check the duct layout and choose a larger fan when the duct is long or restrictive.
Is a 100 CFM fan too large for a bathroom?
Not necessarily. It may be suitable for a bathroom around 100 ft² or for a smaller bathroom with a difficult duct route. Check noise, replacement-air provision and manufacturer performance data.
Can I choose an exhaust fan using air changes per hour?
Yes, especially for a general room exhaust calculation. For a normal bathroom up to 100 ft², the 1 CFM per square foot method is usually easier. For a kitchen, a range hood should be sized for source capture rather than only room ACH.
Should a dining room have an exhaust fan?
A normal dining room usually does not need a dedicated exhaust fan. Where it is open to a kitchen, first provide an effective hood over the cooking appliance. A general exhaust fan may help with heat and background odour, but it should not replace local kitchen extraction.
Does a bigger duct make a fan quieter?
A correctly sized, less restrictive duct can help the fan move air more efficiently and may reduce turbulence. However, sound also depends on fan design, speed, mounting and grille selection.
How long should a bathroom exhaust fan run after a shower?
The required time depends on room moisture, fan airflow and weather conditions. A run-on timer or humidity control can keep the fan operating until moisture has reduced instead of relying on the user to remember.
Can I vent a bathroom fan into the ceiling space?
No. Moist air should be ducted outdoors. Discharging into a ceiling or attic can cause condensation, mould and damage to the building fabric.
Technical References
- Home Ventilating Institute — Bathroom Ventilation
- Home Ventilating Institute — How Much Ventilation Do I Need?
- ENERGY STAR — Ventilation Fans Key Product Criteria
- US Department of Energy Building Science Education — HVAC Kitchen Exhaust
- ASHRAE — Standards 62.1 and 62.2
Technical note: This guide provides practical residential selection guidance. The final fan, duct and electrical installation must follow the product manufacturer’s instructions and the building, mechanical and electrical requirements applicable to the property.
