BATHROOM FAN SIZING

Home Ventilating Institute (www.hvi.org) recommends that a bathroom exhaust fan be able to provide 8 air changes per hour. To achieve this not only must the bathroom size be taken into account but also the resistance created by the length and design of the exhaust duct connected to the fan. To calculate the bathroom size use the following formula:

Step 1: (length x width x height)
Result: This equals the cubic footage of the bathroom

To determine whether the exhaust duct configuration will have an impact on the size of the fan needed you should calculate the equivalent duct length using the following four steps.

Step 2: Measure the overall duct length
Step 3: If using flex duct multiply the length by 1.25
Step 4: For each elbow add 15 feet
Step 5: Add 30 feet for the roof jack or wall cap
Result: This equals the equivalent duct length.

The table below may help you determine which size bathroom exhaust fan is suitable for your particular application. When the exact cubic footage or equivalent duct length is not available round up to the next higher value.

BATHROOM FAN SIZING CHART*
Bathroom Size
Equivalent Duct Length (feet)

(L x W x H)

10 ft.
20 ft.
30 ft.
40 ft.
50 ft.
60 ft.
70 ft.
80 ft.
90 ft.
100 ft.
400 cu. ft.
60 cfm
60 cfm
60 cfm
60 cfm
60 cfm
60 cfm
60 cfm
60 cfm
60 cfm
60 cfm
480 cu. ft.
60 cfm
60 cfm
60 cfm
60 cfm
60 cfm
70 cfm
90 cfm
90 cfm
90 cfm
90 cfm
560 cu. ft.
70 cfm
70 cfm
90 cfm
90 cfm
110 cfm
110 cfm
110 cfm
150 cfm
150 cfm
150 cfm
640 cu. ft.
90 cfm
90 cfm
90 cfm
90 cfm
110 cfm
110 cfm
110 cfm
150 cfm
150 cfm
150 cfm
730 cu. ft.
90 cfm
90 cfm
110 cfm
110 cfm
110 cfm
150 cfm
150 cfm
150 cfm
190 cfm
190 cfm
800 cu. ft.
110 cfm
110 cfm
110 cfm
150 cfm
150 cfm
150 cfm
190 cfm
190 cfm
190 cfm
190 cfm
1,120 cu. ft
150 cfm
150 cfm
190 cfm
190 cfm
190 cfm
190 cfm
190 cfm
190 cfm
190 cfm
190 cfm
1,440 cu. ft
190 cfm 190 cfm 190 cfm 290 cfm 290 cfm 290 cfm 290 cfm
290 cfm
290 cfm
290 cfm
1,920 cu. ft
290 cfm 290 cfm 290 cfm 290 cfm 290 cfm 290 cfm 290 cfm
380 cfm
380 cfm
380 cfm
2,400 cu. ft
290 cfm 380 cfm 380 cfm 380 cfm 380 cfm 380 cfm 380 cfm
380 cfm
380 cfm
380 cfm
* adapted from Panasonic's Fan Sizing Chart (www.panasonic.com)
 

Note that properly sizing a bathroom fan is not, by itself, sufficient to address moisture in a bathroom. The fan must also run long enough to exhaust the moisture. Home Ventilating Institute recommends that bathroom fans be allowed to run for 20 minutes after use of the bathroom. One way to achieve this is through the use of a timer.

It is often adviseable for homeowners to consult with a ventilation contractor to insure proper fan sizing, selection, and installation. Alternative methods of calculating fan sizes may be found on both Panasonic's and Home Ventilating Institute's web sites.