Bathroom Fan CFM Calculator (Free)
Size your bathroom exhaust fan in minutes. Enter your room size and fixtures to get a recommended airflow in CFM (imperial) plus L/s and m³/h (metric). This tool uses common rules of thumb for bathrooms. Always check manufacturer guidance and local building codes.
How to use
- Choose units (Metric or Imperial).
- Enter room dimensions (length × width). If your ceiling is higher than standard, enter the height.
- Set fixtures (toilet, shower, tub, whirlpool) and minimum CFM if you have a target.
- (Optional) Add duct length and elbows to suggest a higher fan rating for losses.
- Click Calculate to see the recommendation.
Worked example
Bathroom: 2.4 m × 1.8 m (≈ 4.32 m²), 2.4 m ceiling, fixtures: toilet + shower.
- Area method: 4.32 m² ≈ 46.5 ft² → ≈ 47 CFM
- Fixture method: toilet (50) + shower (50) → 100 CFM
- Recommendation takes the higher value → 100 CFM (≈ 47 L/s, ≈ 170 m³/h)
Bathroom fan sizing calculator
FAQs
How many CFM do I need?
A common rule is about 1 CFM per square foot of floor area, with typical minimums applied. If your bathroom has multiple fixtures, sizing by fixtures often yields a higher (better) target.
Do high ceilings need a bigger fan?
Often yes. This tool scales the area method if your ceiling is much higher than standard (~2.4 m / 8 ft). Always confirm with manufacturer guidance.
What if my duct run is long or has many bends?
Long runs and elbows reduce delivered airflow. Use the duct allowance to bump the suggested fan rating, and prefer smooth, short, straight ducting.
Is a higher CFM always better?
Higher CFM can be louder and may require specific duct sizes. Balance airflow with noise (sones) and follow local codes.