Size a bathroom exhaust fan. Enter the room dimensions to get the minimum airflow (CFM) and the fan size to buy.
Recommended Fan
— CFM
Round up to the next standard size.Usage Tip
Run the fan during the shower and for about 20 minutes after. A quiet fan (1.0 sone or less) on a timer or humidity sensor actually gets used — the cheapest loud fan usually does not.
THE MATH
Area method: CFM = floor area (sq ft), minimum 50
Air-change method: CFM = area × ceiling height × 8 ÷ 60
Pick the larger, then round up to the next fan size
Air-change method: CFM = area × ceiling height × 8 ÷ 60
Pick the larger, then round up to the next fan size
The common rule is 1 CFM per square foot of floor with a 50 CFM minimum (HVI). The air-change method targets 8 changes of air per hour.
Fans are sold in set sizes (50, 70, 80, 110, 130, 150 CFM and up); round up.
Fans are sold in set sizes (50, 70, 80, 110, 130, 150 CFM and up); round up.
For bathrooms larger than 100 sq ft, size by fixture instead: about 50 CFM each for a toilet, shower, or tub, and 100 CFM for a jetted tub, added up.
Vent the fan to the outdoors through a dedicated duct — never into the attic or a wall cavity, which causes moisture damage and mold.
Keep the duct short, straight, and insulated; check the fan sone rating for noise, and consider a humidity-sensing or timer switch so it runs long enough.
Vent the fan to the outdoors through a dedicated duct — never into the attic or a wall cavity, which causes moisture damage and mold.
Keep the duct short, straight, and insulated; check the fan sone rating for noise, and consider a humidity-sensing or timer switch so it runs long enough.