About this calculator
Size a bathroom, kitchen or laundry exhaust fan in CFM and L/s against the building code minimums and an air-change check.
How it’s calculated
Part 9 of the building code (9.32) asks for supplemental exhaust of at least 25 L/s (50 CFM) from a bathroom or toilet room and 50 L/s (100 CFM) from a kitchen. We also check air changes per hour: CFM = volume × ACH ÷ 60.
Worked example
With these inputs:
- Room
- Bathroom
- Room length
- 10 ft
- Room width
- 8 ft
- Ceiling height
- 8 ft
the result is:
- Fan size
- 90 CFM
- Building code minimum (NBC 9.32)
- 50 CFM (25 L/s)
- 8 air changes per hour
- 85 CFM
- Suggested duct
- 6" (short, straight run)
Tips from the trade
- Rated airflow falls off with long or flexible ducts. Keep runs short and smooth.
- Duct to outside, never into the attic.
Results are estimates for planning and pricing. Check quantities against your drawings, the manufacturer’s data and the building code adopted in your province before ordering.