Bathroom Exhaust Fan CFM Calculator

How it works

under 100 ft²: CFM = floor area (min 50) · over 100 ft²: 50 CFM per toilet, shower or tub, 100 per jetted tub or steam shower · air-change check: CFM = volume × 8 ÷ 60 · derate ≈ 0.7% per equivalent duct foot beyond 10 ft

A bathroom fan's job is to remove moisture faster than a shower adds it, and the Home Ventilating Institute distilled that into two rules: small rooms need one cubic foot per minute per square foot of floor with a floor of 50, and large rooms are sized by what produces steam — fifty CFM per shower, tub or toilet, double for a jetted tub whose agitation throws off far more vapour. Both correspond to roughly eight air changes an hour, which is why the volume method gives similar answers. As with any fan, the rating is at free air, and the small four-inch ducts on bathroom fans lose a large share of it to friction, so the run should be short and straight and the fan bought a size up. Noise decides whether it gets used at all.

Worked example

An 8 × 10 ft bathroom with an 8 ft ceiling is 80 ft², so the area rule gives 80 CFM and the 8-ACH check 85 CFM; through 10 ft of duct with one elbow about 3% is lost, so a 90–110 CFM fan is right — turning the room over about every 7 minutes. A 12 × 12 ft master bath with a toilet, shower and jetted tub would instead be sized by fixtures at 200 CFM.

Frequently asked questions