📊 Required Airflow
🔊 Max Recommended Sone
Sizing Methods Compared
Full Calculation Table
| Method | Basis | Result |
|---|
How to Use the Bathroom Fan Calculator
Enter the bathroom's floor area. This drives the area-based method, which is the usual approach for smaller bathrooms.
Pick intermittent if the fan runs on a switch or timer, or continuous if it runs at low speed all the time as part of a whole-house strategy.
Tap each fixture present for the fixture-based method, and choose how close the fan is to a bedroom for a recommended sone ceiling.
Click Calculate Fan Size to compare both sizing methods and see the recommended CFM, L/s, and sone rating to look for.
Sizing a Bathroom Exhaust Fan in Canada
Three separate numbers decide how a bathroom fan is sized, and it helps to keep them apart. The Canadian code sets a minimum fan capacity. The Home Ventilating Institute (HVI) gives a sizing rule based on floor area or fixtures. The fan's sound rating decides whether anyone keeps running it. This calculator applies all three.
What the Code Requires
Part 9 of the National Building Code sets the bathroom exhaust fan rate at 25 L/s (about 53 CFM) for a fan that runs on demand and 10 L/s (about 21 CFM) for one that runs continuously. ASHRAE 62.2 uses round numbers of 50 CFM and 20 CFM for the same two cases, so the Canadian figures sit slightly above them. These rates are floors, not sizing targets, and they do not change with bathroom size. Provinces adopt and amend the code on their own schedules, so confirm the edition in force with your authority having jurisdiction.
The Area Method
For a bathroom up to 100 square feet (9.3 m²), HVI recommends an exhaust fan rated at 1 CFM per square foot, which works out to about eight air changes per hour. This calculator never goes below the code floor, so a 4 m² (43 ft²) powder room still gets a 53 CFM fan. A 6 m² bathroom is 64.6 ft², so the area method gives 65 CFM (30.5 L/s), which is above the floor and therefore governs.
The Fixture Method
HVI switches to fixtures once a bathroom passes 100 square feet: 50 CFM for each toilet, shower or bathtub, and 100 CFM for a jetted tub. The values are added together, not replaced by the largest one. A 12 m² (129 ft²) bathroom with a toilet, shower and separate tub comes to 150 CFM (70.8 L/s). Large bathrooms often have exterior walls and big windows, and any moisture the fan misses ends up on those cold surfaces, so it is worth running the condensation risk calculator for the room as well. For bathrooms under 100 square feet the calculator still shows the fixture total, but only for reference. If you tap no fixtures in a large bathroom, it falls back to the code floor, so select what is installed.
Continuous Fans
A fan that runs all day only needs the continuous floor, 10 L/s (about 21 CFM) in Canada. Many continuous fans also have a high-speed setting for showers, and that setting should be sized like an on-demand fan, which is why the calculator shows the intermittent figure alongside the continuous one. An on-demand fan should keep running for about 20 minutes after the room is used, so a timer or humidity sensor is worth having.
Sone Rating and Why People Skip Running Their Fan
A fan that is too loud gets switched off, which defeats the sizing exercise entirely. The ENERGY STAR Certified Homes checklist, which builds on ASHRAE 62.2, recommends 3 sones or less for an on-demand fan and 1 sone or less for a continuous one, and ENERGY STAR certified fans up to 200 CFM must come in at 2.0 sones or less. The calculator uses 3.0 for an isolated bathroom, 1.5 for one near a bedroom and 1.0 for continuous operation. The 1.5 figure is a design preference between the two reference points, not a code limit. The sone calculator helps put a published sone figure in context.
Duct Run Matters Too
The CFM on a fan's box is measured at a light duct load of 0.1 in. w.g. (25 Pa). ENERGY STAR requires a bathroom fan to still deliver at least 70% of that airflow at 0.25 in. w.g. (62 Pa), which is closer to a real house with a duct, elbows and a wall cap. A fan rated at 100 CFM that only just meets that line moves 70 CFM at 0.25 in. w.g. When the duct is long or has several bends, choose a fan with some margin over the calculated rate.
Where This Fits Into the Bigger Picture
A bathroom fan satisfies local, point-source exhaust, not whole-house ventilation. If several point-source fans run at once, they can also unbalance a home's pressure, especially homes with an HRV or ERV. Check that with the balanced ventilation calculator, and confirm the home's overall continuous rate with the whole-house ventilation calculator. For a kitchen range hood instead of a bathroom fan, use the kitchen exhaust calculator.
Frequently Asked Questions
For a bathroom up to 100 square feet (9.3 square metres), the Home Ventilating Institute recommends 1 CFM per square foot of floor area, which is about 8 air changes per hour. Bigger bathrooms are sized by fixture instead: 50 CFM for each toilet, shower or bathtub and 100 CFM for a jetted tub. In Canada the fan also cannot fall below the code rate of 25 L/s (about 53 CFM) for an on-demand fan, so the answer is never lower than that. The exhaust fan sizing calculator covers other room types.
An intermittent fan runs when someone switches it on, ideally for about 20 minutes after the room is used, and the Canadian code rate for it is 25 L/s (about 53 CFM). A continuous fan runs at low speed all day, so its code rate is lower at 10 L/s (about 21 CFM). ASHRAE 62.2 uses 50 CFM and 20 CFM for the same two cases. Many continuous fans add a high-speed setting for showers, and that setting is sized like an intermittent fan.
The ENERGY STAR Certified Homes program, which builds on ASHRAE 62.2, recommends 3 sones or less for a fan that runs on demand and 1 sone or less for one that runs continuously. ENERGY STAR certified bathroom fans up to 200 CFM must also be 2.0 sones or less. For a bathroom next to a bedroom, a quieter fan around 1.5 sones is a practical target, since a fan that annoys people gets switched off. Lower sone ratings usually cost more, so match the rating to how the space is used.
Not on their own. A standard bathroom fan that runs only while the room is in use is local exhaust, aimed at moisture and odour at the source, while whole-house ventilation is a separate continuous requirement under the National Building Code and ASHRAE 62.2. A bathroom fan can serve both roles only if it runs continuously and meets the whole-house airflow and sound requirements. Check the home's overall rate with the whole-house ventilation calculator.