Kitchen and Bath Ventilation: Sizing It So It Actually Works

Kitchen and bathroom exhaust fans are sized in CFM (cubic feet per minute), and undersizing is common because a fan that runs and makes noise looks like it's working even when it's moving far less air than the room needs. Sizing and ducting both matter — a well-sized fan with bad ducting performs like an undersized one.

Range hood sizing

A common baseline is 1 CFM per square foot of kitchen area for a standard hood, with adjustments for cooking intensity and hood type. Higher-BTU professional-style ranges and heavy wok or high-heat cooking need meaningfully more airflow than the general baseline.

Bathroom exhaust fan sizing

A widely used baseline is 1 CFM per square foot of bathroom floor area, with a minimum of around 50 CFM even for very small bathrooms, and higher CFM for bathrooms with a separate toilet room, large soaking tub, or steam shower.

• Standard bathroom (up to ~100 sq ft): CFM roughly equal to square footage, minimum 50 CFM.
• Larger bathrooms or those with multiple fixtures: add roughly 50 CFM per toilet, shower, and tub as a rough guideline, since some codes calculate per-fixture rather than per-square-foot for larger rooms.
• Look for a fan rated at or slightly above the calculated CFM need — undersizing is a common and easily avoided mistake.

Ducting mistakes that quietly kill performance

• Long duct runs with too many bends, which add resistance a fan's rated CFM doesn't account for — rated CFM assumes an ideal, short, straight run.
• Undersized duct diameter for the fan's rating, forcing air through a bottleneck.
• Flexible ducting installed with sagging or kinks, which restricts airflow far more than smooth rigid ducting.
• Venting into an attic or soffit instead of straight outside — this dumps moisture into the attic where it can cause mold and rot, and is a common code violation.
• No backdraft damper, letting outside air (and sometimes pests) into the ducting when the fan isn't running.

Signs your current ventilation is undersized or poorly ducted

• Persistent mirror fogging or condensation on the bathroom ceiling well after a shower, even with the fan running.
• Cooking odors and grease film lingering in the kitchen and adjacent rooms.
• Visible moisture or mold near the bathroom fan's exterior vent cap or in the attic near the ductwork.
• A fan that sounds like it's straining or has noticeably weakened airflow compared to when it was new.

A simple test for a bathroom fan: hold a single square of toilet paper against the grille with the fan running — if it doesn't hold with a firm suction, airflow is likely inadequate for the room, whether from an undersized fan, a clogged duct, or a bad duct run.