About this calculator
Size a bath, kitchen or laundry exhaust fan in CFM from the room size and common code minimums.
How it’s calculated
Bathrooms: 1 CFM per sq ft (50 CFM minimum). Kitchens: 100 CFM minimum intermittent. 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
- Code-style minimum
- 80 CFM
- 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 loft or attic.
Results are estimates for planning and pricing. Check quantities against your drawings, the manufacturer’s data and local building codes before ordering.