What minimum size (CFM) vent fan do I need per cubic foot of bathroom?

Consider a bathroom that is 10ft. X 4ft. X 6ft(height). What is the minimum throughput rating for a vent fan? I want a vent fan that truly performs when it comes to removing moisture, human methane, etc.

Does the height of the duct, from attic to roof, factor in choosing a properly rated vent fan?


Calculating Cubic Feet

The first step in determining what size exhaust fan is needed, is to calculate the volume of the room. To do this, you'll simply multiply the length of the room times the width of the room time the height of the room.

  • Length = 10 ft.
  • Width = 8 ft.
  • Height = 8 ft.

10 ft. * 8 ft. * 8 ft. = 640 ft.3

Calculate Equivalent Duct Length (EDL)

The next step is to measure the length of the duct run, and then apply some additional factors to determine the equivalent duct length of the run. For example, if we had this situation.

  • Duct type = Insulated Flex.
  • Duct length = 15'.
  • Duct diameter = 4".
  • Number of elbows = 2.
  • Number of wall caps = 1.

We'll use these numbers, and the chart below to determine the EDL.

EDL Values

So the example above would look like this.

15' of 4" insulated flex duct x 1.5 = 22.5'
4" elbow adds 15' x 2 = 52.5'
1 4" roof cap adds 30' = 82.5'

Which means in our example, the EDL is 82.5'.

Determine Required CFM

Exhaust fans are sized using Cubic Feet Per Minute (CFM), so you'll have to use the two values calculated above to determine the size of the required fan. To do this, you'll use the chart below (you'll always round up).

CFM Chart

Using our example numbers, we'll place our left finger on 640 cu. ft.. Then place our right finger on 90 ft. (because we have to round up). When we slide our left finger to the right, and our right finger down. we'll see that we need a fan rated for at least 150 cubic feet per minute.

Running The Fan Long Enough

Selecting an appropriately sized fan, is not the only thing to worry about. You also want to make sure that the fan runs for long enough, so that the moisture can be completely exhausted. It's often recommended to run the fan for between 20 - 30 minutes, after a shower. This is often made easier by installing an in wall timer to control the fan.

Timer Switch

When selecting a timer, make sure it's rated for motor (inductive) loads and does not say "Incandescent only".

Everybody Wants It Quiet

In my opinion, getting the hot moist air out of the building is the highest concern. Though it would seem, some tend to like it quiet while the air is cleared.

You'll find that exhaust fan loudness is measured in Sone, where the lower the value the quieter the fan. If quiet is important to you, you'll want a fan at or below 1-2 sone. Here is a chart from Panasonic, that might help put sones into perspective.

Sone Chart

  • 4
    This is a beautiful answer! I would like to add some emphasis on a couple of points. (A) Notice that adding caps and elbows adds a disproportionate amount of duct length. It behooves the homeowner/duct installer to carefully plan the run to exclude excessive elbows. (B) Having the fan on a timer switch will help with the issue of running the fan long enough. On a side note - another issue with fan selection is noise. Some people like quiet, others like loud as it tends to mask some other noises that occur at the WC. – Paul Dec 17 '12 at 15:04
  • On the subject of fan noise: Fans should have a noise rating in sones specified somewhere. If not, they are likely to be a relatively noisy fan. IMO, for bathroom fans, 3 sones is pretty quiet, 8 or 9 sones is rather noisy. – bcworkz Dec 17 '12 at 23:38
  • 1
    Note that if you get a Humidity Sensing Bath Fan, you can skip the timer: the fan will run until the bathroom is dry and shut itself off automatically. – Philip Ngai Mar 29 '13 at 20:38

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