The Panasonic website has a lot of info on fan sizing. It isn't always the sq. footage, but also duct type and length and elbows...etc.

I always thought our bath fan did its job...it certainly made enough noise, but when I put a Kleenex up to it only one of four sides was actually sucking up air.

-Babka

Building Code Requirement:

Minimum ventilation for the bathroom is to be a window of at least 3 sq. ft. of which 50% is operable, or a mechanical ventilation system of at least 50 cubic feet per minute (cfm) ducted to the outside. (IRC R 303.3, IRC M 1506.3)

Heating Ventilation Institute (HVI) Recommendations:

HVI is an industry association of heating and air conditioning product manufacturers that, among other things, tests and certifies ventilation products and publishes consumer guides on ventilation issues. These are the HVI recommendations for venting bathrooms:

Small Bathrooms: HVI recommends that in small bathrooms, up to 100 square feet of floor space, the exhaust fan be sized to provide a ventilation rate of 1 cfm per square foot (about eight air changers per hour). This usually results in a ventilation requirement of less than 50 cfm, so the code minimum fan size must be used.

Example 1:
The bathroom is 5' x 9' (with 8' ceilings). Multiply 5x9 = bathroom area of 45 sq. ft. At 1 cfm per square foot the minimum recommendation is a fan rated at 45 cfm. This is less than the code minimum 50 cfm, which must be used.

Medium and Large Bathrooms: For bathrooms larger than 100 square feet in area, HVI recommends a ventilation rate based on the number and type of fixtures present, according to the following table:

Toilet 50 cfm
Shower 50 cfm
Bathtub 50 cfm
Tub/Shower Combination 50 cfm
Whirlpool Tub 100 cfm

To calculate the minimum fan rating, add the cfm for each fixture in the room to arrive at a total cfm.

Example 2:
The bathroom is 20'x12'. There is a tub, a separate shower enclosure and a toilet.
Toilet 50 cfm
Shower 50 cfm
Bathtub 50 cfm
Minimum Fan Rating 150 cfm

Toilet Compartment: If a toilet is in an enclosed stall with a door, then the toilet enclosure is considered by most code authorities to be a separate room. Likewise, if a shower is enclosed right to the ceiling, it will be considered a separate room. These guidelines recommend a separate mechanical exhaust system for "each enclosure", which means a fully enclosed toilet or shower must have its own exhaust fan separate from that of the rest of the bathroom. In most localities, this is also the building code requirement. A steam shower always needs its own ventilation. Strictly follow the manufacturer's recommendation for venting a steam shower. Otherwise you will have mold growing in places you would not think mold could grow.

Example 3: The bathroom is 15'x15'. There is a tub, a separate, fully enclosed, shower and a fully enclosed toilet in a stall with a door.

The fully enclosed toilet and fully enclosed shower should have their own exhaust fans rated at 50 cfm each. The bathroom fan will exhaust only the bathtub and should also be rated at the code minimum 50 cfm.

How Long to Ventilate: When house air is exhausted to the outside, heat goes with it, meaning that your furnace has to work harder to make up for the lost heat. You want to exhaust moist air to reduce the risk of mold and mildew, but once the moist air is gone, you want to stop ventilating. HVI recommends that the fan be left on for a minimum of 20 minutes after use of the bathroom. A timer is a good solution, allowing the fan to turn off automatically at the proper time. There are also fans on the market now that sense the level of humidity in the air and automatically turn on when it is too high, and turn off when it is back to normal. Unfortunately, there is no fan that senses odor, so a manual switch is still necessary.

Calculate the air movement necessary for proper ventilation.

To determine CFM requirements, determine the size of the room. Measure and multiply the length, width, & ceiling height of the room to determine cubic footage. Use the multiplication factor of 0.13 and round up to the next "tenth" (.10).