How are Bathroom Exhaust Fans Rated: Guide for homeowners

Knowing what all the numbers mean on the box is vital to making a well-informed decision, no matter if the purchase is a TV or a bathroom fan.

Since I recently had to purchase a new bathroom fan I needed to educate myself on their specifications. I will save you the trouble of navigating dozens of boring user manuals and let you know what are the most important things I learned.

Bathroom fans are rated by two main categories: how much air they move(CFM) and how much noise they make(Sones). When comparing fans of similar size, a fan with higher CFM and lower Sones is better.

Ok now you know how are they rated, but what do the terms mean? What is a good value to look for? Let us dive in and figure it out.

What is a CFM rating?

This is the most important value to look for when choosing a new bathroom fan or any fan for that matter. It needs to be adequate for your needs. If you install a tiny 50 CFM fan in a massive 200 square feet bathroom it will be of little use in keeping the rooms moisture level down and room smelling fresh.

Cubic Feet Per Minute(CFM) is used to measure the amount of air a fan moves in one minute. The bigger the number the more airflow the fan will provide.

Think of it this way: let’s say you have an 8 ft high 50 sqft bathroom. That is 400 Cubic feet. (8ft x 50ft =400cu. ft.) A fan that replaces the air once an hour in our hypothetical bathroom is rated at 6.66 CFM (400cu. ft. / 60 minutes). However once an hour is not nearly enough, this example was presented only to clarify the concept of how they are rated.

How many CFM do I need for a bathroom fan?

The Home Ventilating Institute recommends 8 air changes in an hour.

However If you want to keep things simple and refrain from any calculations there is a very easy and reliable way to figure out how powerful fan you will need. You can use the total area of your bathroom in square feet as a guide. In most cases this will be very accurate.

You can use the total area of your bathroom in square feet as a guide.

50 sq. ft. Bathroom = 50 CFM
100 sq. ft Bathroom = 100 CFM

If you are anything like me and want to calculate which fan will provide exactly 8 air changes per hour then keep reading, I’m about to overcomplicate a very simple thing.

After calculating(50x8x8/60) I came to the conclusion that our imaginary 50 sq. ft. bathroom from before would call for a fan that is rated at 53.33 CFM. The method of just getting one with the same CFM as your square footage comes very close to this number. This proves two things – first that by going by the sq. ft. is reliable and that I am needlessly overcomplicating this…but I will continue.

50x8x8/60 = 53.33 CFM

  • 50 = Square footage
  • 8 = Ceiling height
  • 8 = Recommended air changes per hour
  • 60 = 60 minutes (This will convert cubic feet per hour to cubic feet per minute (CFM)

What is a Sone Rating?

Let’s start by saying that if you are a person who is sensitive to background noises, then understanding the sone rating can really be beneficial. Furthermore sone rating is used for countless other home appliances to measure how noisy they are.

Sone rating is used to measure how much sound a fan makes when operational. The Sone rating is linear, meaning 2 sones is about twice as loud as 1. The rating is based on how it is sensed by the average listener.

Here I have a list of 8 super quiet bathroom fans. They are truly the quietest on the market today and I will keep this list updated.

If you are picking out a new bathroom fan then consider choosing one with the lowest sone rating that your budget allows. However keep in mind that the CFM still needs to be adequate for the room size.

There are bathroom fans with a sone rating of 0.3. They are close to silent. And well worth the premium.

Let’s look at the sones levels a little closer to give you a better idea how loud or quiet they really are.

Sone Levels

  • 0,3-1 Sones Most likely you are not gonna even notice that the fan is turned on. You will have to pay a premium for a fan with that low sone rating, and they tend to be very low CFM as well.
  • 1-2 Sones This can be compared to the sound of rustling leaves. This won’t bother most people with people who are not hypersensitive to background noises.
  • 2-2.5 Sones When looking at fans that are 2 sones and up you will start to hear when it is turned on, you can compare it to a working refrigerator. However it should not be noticeable in the next roof if you close the door.
  • 2.5-3 Sones This will be 3 times as loud as a 1 sone. Can be compared to a dishwasher.
  • 3-4 Sones Sound level comparable to when watching TV with normal volume. This is too loud for a bathroom fan and should be avoided if possible. It will be heard in the next room even when the bathroom door is closed.

How do Sones Compare to Decibels?

This table will help you compare sones to decibels. Note that the values are approximate since it is not possible to directly calculate them.

You can find a Sone to Decibel calculator here

Sone to dbA conversion chart

Duct size affects noise level

The sone rating on the box is measured when connecting to the correct size duct. For example if you have 3 inch ducting in your bathroom and your new fan comes with a 4-inch connection it will be louder than stated in the specifications.

Although it is still possible to use this fan with an adaptor it will be considerably noisier than 0.5 sones since it will have to work much harder to push the same amount of air through the smaller duct.

It might seem like there won’t be much of a difference between a 4 inch an 3 inch duct. By increasing the diameter by one inch from 3 to 4 inches increases the area by 77%. Check out the image below.

Other things to consider

Although CFM and sone rating are the main things you need to look for there are other things to consider.

Bathroom Fan Housing Size

The most important thing is the actual physical size of the fan. By replacing it with a fan that is similar to the one you have now you save yourself a lot trouble.

It is best to measure the opening and find a fan with a similar size housing. Bathroom fans are not standard size as I discuss in my previous article.

Airflow efficiency (CFM Per Watt)

This is something that actually is not commonly marked on the product description for bathroom fan and you have to calculate on your own.

It is measured by Cubic Feet Per Minute Per Watt. It can be calculated by dividing the CFM by wattage. Higher value = more efficient.

You can calculate this by dividing the CFM by wattage

For example, a 50 CFM fan with a 6.5 Watt motor has an airflow efficiency rating of 7.69 CFM Per Watt.

WattCFMCFM per watt (Higher is better)

Electricity use

This is rated in watts (W). The bigger the value the more electricity the fan uses. I have an entire article that is dedicated to bathroom fan electricity use, so I won’t discuss it here in any more detail.

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Joonas Pütsepp

I like it when I'm able to fix everything that needs fixing around the house. In order to do that, I have to do a lot of research. This site will cover everything I learn and maybe help others do the same.

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