There’s nothing worse than walking into a bathroom, and the exhaust fan is so weak it can barely move the air at all. Or the exhaust fan is so powerful that it is incredibly loud. This is why, when it comes to buying a new bathroom exhaust fan, it’s important to size your exhaust fan correctly. Bathroom exhaust fans come in a wide variety of sizes so getting the measurements right is important. When sizing a bathroom exhaust fan, however, we don’t mean the physical dimensions of the fan but its CFM rating. Today we are going to look at how to calculate the correct CFM for your bathroom, what factors affect the CFM, and Martec’s handy exhaust fan calculator. So let’s discuss how many CFM you need for your bathroom exhaust fan.
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A bathroom extractor fan is mandatory in bathrooms without a window. But we also recommend them in bathrooms with windows. The exhaust fan removes moisture and humidity from the bathroom, helping to prevent damage to appliances, walls, and fittings, as well as the growth of mould. A bathroom exhaust fan must be strong enough to cycle fresh air into the bathroom 8 times an hour.
Fans that are too small will not be able to remove moisture from the room. This can cause warped cabinetry, foggy mirrors, and mould or mildew growth. A fan that is too big can cause a bathroom to be cold and noisy. This is why calculating the correct CFM is important.
CFM stands for Cubic Feet per Minute, and it essentially tells you how much air the bathroom fan can move in a minute. Most bathroom exhaust fans come in a CFM rating range of 50CFM (4.6 square meters) to 30CFM (2.7 square meters), but they can go as high as 200CFM (18.5 square meters) or more. This rating should be listed somewhere on the packaging of the exhaust fan. Some stores even organise their products based on the CFM.
There are some different ways to calculate the CFM for your bathroom, but all measurement methods should aim for at least 8 air changes per hour. As we mentioned above, this means the bathroom’s air volume should be totally replaced 8 times in 60 minutes.
There are four ways to size a bathroom exhaust fan. These calculations are based on the floor area, ceiling height, and any additional enclosed spaces – like the shower – in the bathroom.
The most common way to size a bathroom fan is using the calculation of 1CFM per square foot of floor area. To work out the floor area, simply take a tape measure and multiply the length and width of the bathroom (including shower and bath areas). So if you have a bathroom that is 80 square feet (7.4 square meters), then you will need a bathroom exhaust fan that is at least 80CFM.
This calculation is used for “regular-sized” bathrooms of 100 square feet (9.2 square meters) or less. If you have a very small bathroom of less than 50 square feet (4.6 square meters), you will want to use a minimum of 50CFM or slightly more. However, we recommend never going more than 10-20% large than the minimum CFM. We also recommend not going under the minimum CFM as the exhaust fan will be underpowered and may not adequately remove the moisture needed.
If you have a standard sized bathroom with high ceilings, you will need to calculate the CFM a little differently. The standard ceiling height for most bathrooms is 8 feet(2.4m), so if your ceiling is 10 feet (3m) or more, you will need to factor in the higher vertical space. To work out the CFM needed, multiply the floor square foot by the ceiling height. Then, divide this number by 60. Finally, multiply by 8.
As an example, if you have a bathroom that has a 120 square foot (11.14 square meter) floor and 10-foot (3m) ceilings:
120 x 10 = 1,200
1,200 / 60 = 20
20 x 8 = 160CFM
So in this scenario, you would need a bathroom fan of at least 160CFM or two fans that equal 160CFM.
When it comes to larger bathrooms, over 100 square feet (9.2 square meters), we recommend you calculate the CFM according to the number of plumbing fixtures, rather than the floor size. Assign 50CFM to each fixture, i.e., toilet, shower, bath, etc. Larger fixtures, like spa baths, should be assigned 100CFM.
So for a bathroom with a toilet, shower, and bath:
Toilet – 50CFM
Shower – 50CFM
Bath – 50CFM
=150CFM
In this instance, the minimum CFM for this bathroom would 150CFM.
If your bathroom area has an enclosed toilet or shower, you will need to purchase a second bathroom exhaust fan for that area. Because this enclosed area has a door, it will prevent the fan from ventilating the space properly. And since these spaces are so small, a small 50CFM bathroom exhaust fan is recommended.
To make choosing your bathroom exhaust fan easier, the team at Martec have developed a calculator to work out the correct exhaust fan based on your room size! Start by selecting the space for your exhaust fan from the drop-down menu: Bathroom, Laundry, or Utility Room. From there, measure your room and enter the room width, length, and ceiling height in metres, in the correct boxes. Explore Martec’s exhaust fan calculator here!
The calculator will then give you the minimum and maximum airflow in m3/hr. You can then explore the Martec range to find the perfect exhaust fan for your bathroom. Still have questions? Contact the team at Martec today!
Bathroom exhaust fans are required by the NEC (National Electrical Code) and local building codes for any bathroom that doesn't have a window that can be opened to provide ventilation. In some jurisdictions, they are required even if windows are present. Bathroom exhaust fans must be vented to the outside through ductwork or sidewalls.
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Installing a bathroom exhaust fan is worth the time and effort because it removes hot, moist air and odors. Moist air leads to mold growth and odors lead to general unpleasantness.
But installing a weak, undersized exhaust fan is almost not worth the effort, even if the fan meets the minimum requirements. It's important to understand how vent fans are sized and how to choose one with the proper capacity for your space.
Types of Bathroom Exhaust Fans
If you have to replace a bathroom exhaust fan, your best bet is to go with the type that's already in place. Four types of bathroom exhaust fans include:
Modern ceiling-mounted vent fans come in variations that may integrate ceiling lights, heat lamps, or sometimes even Bluetooth speakers. Accessories can generate extra heat in a bathroom, though not as much as older fans without all the bells and whistles. Check with the manufacturer to make sure you can put insulation over the box of a fan with accessories.
Standard Fan Sizing
Bathroom vent 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 six feet wide and nine 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.
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When sizing a vent fan, a factor to consider is duct size and length. Most 50 CFM fans will run well with a 4-inch round duct. But as you get up into the higher CFM fans, duct size will have to be increased to a 5- or 6-inch round duct. The length of the run, as well as the number of fittings and elbows, will also affect the amount of air your duct can effectively carry.
There are somewhat complicated formulas for calculating proper duct size for different length runs and configurations, but the instructions that come with the fan will outline the requirements. Just make sure there is sufficient space for the ductwork in the area where you're installing the fan. Trying to force too much air through an undersized duct will make the fan work too hard and will provide insufficient venting.
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Sizing for Large Bathrooms
For bathrooms over 100 square feet, exhaust fans can be sized according to the number of fixtures in the room. To calculate using this formula, add up the required CFM ratings for all of the fixtures:
For example, if your bathroom includes just a shower and a toilet, it needs a 100 CFM fan, while a bathroom with a jetted tub, toilet, and shower needs a 200 CFM fan.
Another calculation method with three steps is useful for large rooms with high ceilings (well over 8 feet). In this method, multiply the square footage by the ceiling height, divide by 60 (minutes in one hour), then multiply by eight (recommended number of air exchanges per hour). For example, for a room that's 120 square feet and has a 10-foot ceiling:
Fan Noise Ratings
Once you've chosen the right fan size for your bathroom, consider the sound rating, as demonstrated by the sone rating of each fan. (Sone rating is a measure of loudness.) A quiet fan might be a worthy investment, particularly in a bathroom you'd like to make peaceful and relaxing. The lower the sone number, the quieter the fan will sound when running. A sone rating of "1" is the quietest fan you can buy. Here are other sone ratings compared to other sounds:
A Second Fan May Be Needed
If your bathroom has a toilet or shower area enclosed by a door, it's a good idea to install a separate exhaust fan for that area. If the enclosed space is small, a 50 CFM fan should suffice. Otherwise, use the appropriate calculation based on the room size. Just remember that the fan needs airflow to do its job.
If the door to the enclosure 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. If the gap under the door in an enclosure is less than 5/8 inch, you should leave the door slightly ajar when using the toilet or shower when the enclosure's vent fan is running. Another solution is to install a louvered door that allows airflow.
A bathroom exhaust fan with accessories—especially heat lamps—will likely require its own dedicated circuit. If the vent fan unit is just a vent fan with no add-ons, most local electrical codes will allow it to be powered by the same general lighting circuit that controls the room's lighting fixtures.
FAQ
What is the difference between an exhaust fan and a ventilation fan?
An exhaust fan draws dust, contaminants, and polluted air out and a ventilation fan brings fresh air in, helping it to circulate within the space.
How often should you clean a bathroom exhaust fan?
You should thoroughly clean your bathroom exhaust fan every six months. This includes dismantling the fan and cleaning the vent cover, motor housing, blades, and fan housing. Keep the cover dust-free by vacuuming the vent once a month with a brush attachment.
When should you replace your bathroom exhaust fan?
The average lifespan of a bathroom exhaust fan is around 10 years. If you notice more moisture, an unpleasant odor, and the fan sounds louder than it has in the past, it might be time for a new one. But don't worry, it's a job you can do yourself.
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