What size bathroom exhaust fan do you need?

The rule, in HVI’s words

The Home Ventilating Institute publishes the sizing guidance the industry uses: “For bathrooms up to 100 square feet in area, HVI recommends that an exhaust fan provide 1 CFM per square foot.”

Above 100 sq ft, it switches to a fixture count, with “50 CFM” each for a toilet, a shower and a bathtub, and “100 CFM” for a jetted tub.

Worked examples

• 5 x 8 ft (40 sq ft): 40 CFM. A 50 CFM setting covers it.
• 6 x 9 ft (54 sq ft): 54 CFM. Still a 50 CFM fan, or 80 if the room is enclosed and slow to clear.
• 8 x 10 ft (80 sq ft): 80 CFM.
• 10 x 12 ft (120 sq ft), with toilet, shower and tub: over 100 sq ft, so use the fixture method — 50 + 50 + 50 = 150 CFM. That is more than one 110 CFM fan delivers, which is HVI’s way of telling you a big bathroom wants two fans or one larger unit.

Measure the floor area, not the ceiling height. HVI’s rule is area-based, and it already assumes a normal ceiling.

Bigger is not better

An oversized fan is louder, pulls more conditioned air out of the house, and in a tight home can struggle to draw replacement air through a closed door. Size to the room and put the effort into running it long enough instead: HVI recommends leaving it on “for 20 minutes after use of the bathroom”.

The second number: sones

A fan nobody switches on ventilates nothing, and noise is why people do not switch them on. HVI’s threshold is plain: “For quiet bathroom ventilation the fan should be rated at 1.0 sones or less.”

Panasonic publishes below 0.3 sones at 50 and 80 CFM on the WhisperCeiling DC. Any fan that publishes no sones figure at all is telling you something by omission.

Read the conditions on the figures

Panasonic publishes its sones and watts at stated static pressures — 0.1 in. and 0.25 in. w.g. — because a fan works harder against a long or crushed duct. Two fans with the same CFM on the box can behave very differently once the ducting is real.

ENERGY STAR certification is a useful shortcut here, since it requires fans to “meet maximum allowable sound levels (in sones), minimum efficacy levels (in cubic feet of airflow per minute per watt (cfm/W))”.

Then check it fits

The airflow is not the only measurement. Check the housing depth against your joists — Panasonic’s WhisperFit publishes 5-5/8 in. for 2 x 6 joists — and the duct size, 4 in. or 6 in., against what is already in the ceiling. For small bathrooms, the shortlist is here.

If you rent

Fans are Owner-only on our install rating. If you are asking a landlord for a better one, the CFM arithmetic above and the 1.0 sone threshold are the two numbers to put in the message. How to frame that is here.