Bathroom Exhaust Fan CFM Calculator
A bathroom fan is sized in cubic feet per minute, and the right number depends on the size of the room or, in a large bathroom, on what is in it. Enter both and this gives the rating to buy, checked against the ASHRAE minimums.
How this calculator works
- Up to 100 sq ft: the Home Ventilating Institute recommends 1 CFM per square foot, about eight air changes an hour.
- The floors under that: ASHRAE 62.2, as summarized by the Building America Solution Center, calls for at least 50 CFM for a fan run after use, or 20 CFM for one running continuously. Where the intermittent target is 50 CFM, ENERGY STAR recommends a fan rated at least 70 CFM. The largest of these figures governs.
- Over 100 sq ft: HVI switches to a fixture count, 50 CFM each for a toilet, a shower or a bathtub and 100 CFM for a jetted tub, because a large bathroom is not wet everywhere.
- Air changes per hour: the rating times 60, divided by the room's volume.
A rating is only reached on a good duct: smooth rigid pipe or flex pulled tight, the size the fan's outlet asks for, no reducing a 6 in outlet to a 4 in pipe, and a dampered cap that ends outdoors, not in the attic. Two fans never share one duct. For quiet enough to actually use, ENERGY STAR recommends 3 sones or less for a fan run after use and 1 sone or less for a continuous one, and BASC suggests a timer that keeps it running for an hour after a shower (HVI suggests at least 20 minutes). A fan over a tub or shower must be listed for that location and is typically connected to a GFCI-protected circuit; check the model's installation instructions.
Every figure here comes from our guide, Sizing a Bathroom Exhaust Fan in Cubic Feet per Minute, which explains each rule and the exceptions in full.