Everyday
Bathroom Exhaust Fan Size Calculator
Enter your bathroom's dimensions and fixtures, and the page works out the CFM the room requires under the HVI method, then adjusts for your duct run and rounds up to the fan size you should actually buy.
How it works
Behind the numbers
The sizing follows the Home Ventilating Institute method that most codes and manufacturers use. Small and mid-size baths get 8 air changes per hour: multiply length by width by ceiling height, then by 8, then divide by 60. A 5 by 8 foot bath with an 8-foot ceiling needs about 43 CFM by that math, which the IRC minimum of 50 CFM for intermittent ventilation then overrides.
Baths over 100 square feet switch to the fixture method, because one number per room stops tracking where the moisture actually comes from. Each toilet, shower and standard tub counts 50 CFM, and a jetted tub counts 100 because it holds more hot water and runs longer. The calculator takes whichever is larger, the fixture total or the air-change figure, and adds 50 CFM if there is a steam shower.
The number on the box is measured in a test setup, not through your duct. A short rigid run with one elbow delivers close to the rating; elbowing through flex duct can cost a third of it. The recommendation multiplies the requirement by 1.0, 1.2 or 1.4 depending on the run you pick, then rounds up to a standard size (50, 70, 80, 90, 110 CFM and up). Whatever you buy, the sone rating decides whether anyone runs it: at or under 1.0 sone is conversation-quiet, and a fan nobody turns on moves no air at all.
FAQ
Questions, answered
What size exhaust fan do I need for a 5x8 bathroom?
A 5 by 8 foot bath with an 8-foot ceiling works out to about 43 CFM at 8 air changes per hour, so the 50 CFM code minimum governs and a 50 CFM fan is the standard pick. If the duct run is long or flex, step up to a 70 CFM model so the delivered airflow still clears the minimum.
How many CFM per square foot does a bathroom fan need?
About 1 CFM per square foot for baths up to 100 square feet with standard ceilings, which is the same rule as 8 air changes per hour restated. Taller ceilings raise the volume and the requirement with it. Over 100 square feet, drop the per-foot rule and count fixtures instead, at 50 CFM each and 100 for a jetted tub.
Is an 80 CFM fan enough for a primary bathroom?
For a primary bath under 100 square feet, usually yes, with headroom over the 50 CFM minimum. Over 100 square feet, add the fixtures: toilet plus shower plus tub is 150 CFM, and an 80 CFM fan will leave mirrors fogged and moisture lingering. Size by the fixture total and derate for the duct run.
What is a sone, and how quiet should the fan be?
A sone is a loudness rating measured at the fan. One sone is about the hum of a quiet refrigerator; cheap builder fans often run 3 to 4 sones and get switched off for the noise. For a bath used early or late, look for 1.0 sone or less, and check whether the rating is at your fan's CFM, since some are quoted at a lower speed.
How long should a bathroom fan run after a shower?
About 20 to 30 minutes clears the moisture load from a typical shower. A timer switch or a humidity-sensing fan makes that automatic, which matters more than a small CFM difference: a correctly sized fan switched off with the light leaves most of the moisture in the room.