The bathroom exhaust fan is the house’s front line against its most concentrated moisture source, and most of them fail at their job for reasons that have nothing to do with the fan: undersized ratings, strangled ducts, attic terminations, and run times that end with the shower instead of the moisture. Bathroom exhaust fan sizing is the first arithmetic; the ducting and habits are the rest. This reference covers all of it.
Sizing: the CFM arithmetic
The standard rule sizes small baths at one CFM per square foot with a 50 CFM floor: a 5-by-8 bath computes to 40 and installs a 50. Larger rooms (over roughly 100 square feet) size by fixture instead, 50 CFM per toilet, shower, and tub, 100 for a jetted tub, summed; enclosed toilet rooms take their own 50 CFM fan; and steam showers move the conversation to dedicated equipment. Two buying specs ride alongside CFM. Sones, the loudness rating, decide real-world use: fans at 0.5 to 1 sone run unnoticed and therefore run, while the 4-sone roarers of past decades get switched off early, which is a moisture decision disguised as a comfort one. Controls decide duration: timer switches (the 20-to-30-minute post-shower run being the entire point) and humidity-sensing fans automate the habit the mirror keeps requesting.
Ducting: where rated CFM goes to die
A fan’s box rating assumes a short, smooth, straight duct, and every real-world deviation taxes it: undersized duct (4-inch where the fan wants 6), flex duct’s corrugated drag, sags that collect condensate, sharp bends, excessive length, and stuck or bird-blocked exterior dampers. The symptoms are familiar, a fan that roars while the mirror fogs and the tissue test (a square of toilet paper held to the grille should snap flat and hold) fails, and the fixes are mechanical: smooth-wall duct at the fan’s specified diameter, runs short and straight as the framing allows, gentle bends, insulated duct through cold attics so exhausted steam does not condense and rain back through the grille, and a working exterior cap with damper. The termination rule stands above all of it: outdoors, always, through a roof or wall cap, never into the attic, soffit cavity, or joist bay, the attic termination being among the most common and consequential shortcuts in housing, delivering concentrated shower moisture to cold sheathing and growing the attic mold and frost that the ventilation reference then gets blamed for. Soffit terminations earn their own demerit, exhausting steam directly below the soffit intake that inhales it right back.
Habits and maintenance
The run-time habit is the cheap half of performance: on with the shower, off 20 to 30 minutes after, door cracked to supply makeup air, and the timer or humidity control installed so the habit survives the household. The maintenance is a twice-yearly five minutes: grille off and washed, fan blades and housing vacuumed (dust load meaningfully cuts flow and adds noise), damper checked at the exterior cap, and the tissue test repeated as the performance audit. The upgrade math favors action: modern fans move more air at a fraction of the noise and wattage, retrofit into existing housings in many cases, and cost less than the mold cleanup and repainting cycles that a fogging bathroom schedules, with ceiling stains, peeling paint, mildewed grout lines, and condensation reaching the windows as the running audit of a bathroom losing its moisture war.
Beyond the bathroom
The bath fan is one department of the house’s exhaust economy: kitchens carry their own ducted-versus-recirculating question, dryers their own vent discipline, and tight modern houses eventually ask the whole-house ventilation question that spot fans only partly answer. Within its department, though, the bath fan’s brief is complete: size it to the room, duct it like you mean it, terminate it outdoors, run it past the shower, and let the mirror grade the work.
The bottom line
One CFM per square foot with a 50 floor, sones low enough to actually use, smooth full-size duct outdoors and nowhere else, and a timer holding the fan on for the half hour the moisture actually needs. The fan is the cheapest appliance protecting the most rot-prone room in the house, and it only works when the duct and the habit let it.