How to Size a Bathroom Exhaust Fan

Bathroom fan

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Bathroom exhaust fans, or vent fans, do two important things: they remove hot, moist air and they remove odors. Hot, moist air leads to mold growth, and odors lead to general unpleasantness. Building codes require vent fans in all new and remodeled bathrooms, and they're just as important in old bathrooms, even those that have windows for ventilation (which aren't so great in wintertime and never remove moisture as well as fans). But installing a weak, undersized fan is almost not worth the effort, even if the fan meets the minimum requirements. Use the following tips to get the sizing right. 

Standard Fan Sizing

Bathroom exhaust fans are rated for the amount of air they can move, measured in cubic feet per minute, or cfm. Standard fan sizing applies to bathrooms that are 100 square feet or less. The rule of thumb is that you need at least 1 cfm per square foot of room area. To determine the square footage of your bathroom, multiply the length times the width. For example, if your bathroom is 6 feet wide and 9 feet long, its square footage is 54. Therefore, it should have a fan rated for at least 54 ​cfm. But before you start shopping, there are a few things to consider. First, it's a good idea to oversize the fan a bit. In the above example, you should get at least a 60 cfm fan for good measure. Second, if your bathroom has a jetted tub or separate rooms or alcoves, you might want more than one fan (see below). And third, the minimum allowed fan size is 50 cfm, so if your bathroom is, say, only 42 square feet, you still need a 50 cfm fan. 

Sizing for Large Bathrooms

For bathrooms over 100 square feet, exhaust fans can be sized by the number of fixtures in the room. To calculate using this formula, add up the required cfm ratings for all of the fixtures:

  • Bathtub: 50 cfm
  • Jetted tub: 100 cfm
  • Shower: 50 cfm
  • Toilet: 50 cfm 

Therefore, having a shower and a toilet in a bathroom would require a 100 cfm fan, while a bathroom with a jetted tub, toilet, and shower needs a 200 cfm fan. 

Another calculation is useful for large rooms with high ceilings (well over 8 feet): Multiply the square footage by the ceiling height, divide by 60 (minutes in one hour), then multiply by 8 (recommended number of air exchanges per hour). For example, for a room that's 120 square feet and has a 10-foot ceiling:

120 x 10 = 1,200

1,200/60 = 20

20 x 8 = 160 cfm

Do You Need More Than One Fan?

If your bathroom has an enclosed toilet or shower area, it's a good idea to install a separate exhaust fan for that area. If the space is small, a 50 cfm fan should suffice. Otherwise, use the appropriate calculation based on the room size. A separate fan is particularly effective when the enclosed space has a door. Just remember that the fan needs airflow to do its job. If the door is closed and there's no gap at the bottom of the door, the fan will be starved of "makeup" air and will perform poorly. The gap should be at least 5/8 inch tall, or you should leave the door ajar when the fan is running.