How to Size Bathroom Vent Fans

Measure your bathroom's cubic volume (CV). For instance, you need to know how much air exists in your bathroom. You get this information by multiplying length (L) x width (W) x height (H). L x W x H = cubic volume (CV).

Determine your bathroom's cubic foot per minute rating (CFM) Manufacturers rate fans according to how much cubic feet of air they can move in one minute. CFM equals CV divided by eight. Eight equals the Home Ventillation Institute's recommended number of times per hour your air should completely circulate. Therefore, to determine the CFM of the fan you need divide CV by 8 = your bathroom's CFM. For example. 8 (L) x 7 (W) x 8 (H) = 448 (CV). 448 / 8 = 56. You need a fan with a minimum of 56 CFM.

Determine how much air the fan can circulate per minute. Small bathroom fans have CFM ratings of 50 to 60. Moderate-sized fans have 80 to 90 CFM. More powerful fans have 110 to 150 CFM.

Decide if you must calculate custom fixtures into your CFM. For instance, if you have a larger bathroom of 100 square foot or more or if you have special appliances like hottubs or jet pools, you need to consider the humidity of each water-producing appliance. For instance, in a large bathroom, a shower requires 50 CFM fan for proper ventillation. A toilet requires another 50 CFM fan. A tub requires another 50 CFM fan. If you have a jacuzzi or a jet-tub, you need a 100 CFM fan. Choosing individual fans helps circulate air for each appliance. This multi-fan solution provides the best aeration throughout a large bathroom. Alternately, you can purchase one powerful 150 CFM fan to aerate the whole bathroom. Although this strategy works, it doesn't work as efficiently as different fans.