For most bathrooms this works out to one CFM per square foot of bathroom area. For example, a 7' x 10' bathroom would require a 70 CFM fan. A 50 CFM rating is recommended as a minimum for bathrooms 50 sq. ft.
Most fans are rated from 50 to 110 CFM, but some models designed for large master bathrooms, rec rooms and home theaters can be rated even higher. Choose a fan that can ventilate at least 1 CFM per square foot of room. So, for an 80 square foot bathroom, select an 80 CFM fan.
If you live in a neighborhood where all the houses have been built by the same contractor, chances are you have a 50 CFM fan. We recommend at least an 80 CFM for most bathrooms but a 100/110 for a larger master bathroom. This will help move the moisture out of the bathroom and reduce the risk of mold.
To adjust the CFM, you can change the speed of the blower or alter the ductwork size. Increasing the blower speed won't have much effect if a room's duct is too narrow. A 12' x 12' foot room is 144 square feet. You generally need at least 1 CFM per square foot, meaning that room's duct should have a 144 CFM capacity.
For example, if you have a small bathroom with only a shower, you may be able to get away with an exhaust fan that is 60-80 CFM. But if you have a large bathroom with a tub, shower, and multiple sinks, you may want an exhaust fan that is closer to 100 CFM or more.
Chapter 15 of the International Residential Code (IRC) requires bathrooms and toilet rooms to have a minimum local exhaust capacity of 50 cfm intermittent or 20 cfm continuous.
If it's just a small bathroom with a toilet and a shower, you may only need a 100 CFM fan. If you have a jetted tub and shower, you will likely need a 200 CFM fan.
On the other hand, if your CFM is too high for your room size, it can cause ventilation problems. Too much air results in too little ventilation, which in turn causes high humidity. The excess moisture in the air can then lead to mold and mildew, as well as other problems.
How many CFM do I need for a 10x10 room? It is considered that 135 CFM is needed per square foot of space. So 13.5 CFM would be required for a 100-square-foot area. A room of 1,000 square feet would require 135 CFM.
Draw a mark on the bathroom ceiling where you'd like to install the vent fan. For optimum performance, locate it between the shower and the toilet. Use an extra-long, 3/8-inch-diameter spade bit to bore a reference hole through the ceiling and into the attic.
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.
A fresh air fan, also known as a ventilation fan, brings fresh air into a space, while an exhaust fan expels stale air, odours, and excess moisture from a room. Both fans serve different purposes and are used in different areas of a home or office.
Make sure the fan you choose is powerful enough to ventilate your bathroom. The room's size determines the necessary cubic feet per minute (CFM) rating of the fan. The CFM rating measures the volume of air the fan moves. As a general rule, you'll need at least 1 CFM per square foot of space.
Ventilation to the Outdoors (IRC M1503. 1): Residential range hoods should vent to the outside to effectively expel smoke, grease, and cooking odors, maintaining the home's air quality. This requirement is crucial for preventing the recirculation of contaminants within the home.
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.
Ceiling Fan Sizes by Room
Medium: If your room is up to 12' x 14', stick with a fan that is 42" – 54" in diameter. Medium rooms include bedrooms and dining rooms. Large: A room that is up to 18' x 20' should feature ceiling fan sizes 52" – 60" in diameter. This includes family rooms and large bedrooms.
The higher the CFM, the more air the fan circulates, making it more effective at cooling your room. For a standard room with an 8-foot ceiling, a fan with a CFM rating of 4000 to 5000 is generally sufficient. However, for larger rooms or rooms with higher ceilings, you might need a fan with a higher CFM rating.
You can calculate the CFM needed for your system. Start with the total volume of air (in cubic feet). Divide by the exchange rate (how quickly you want to replace the air), and the result is the total CFM you need for your system.
If the CFM is low, a system's capacity for cooling or heating may not be enough for the space. This can result in uncomfortable temperatures throughout your home. On the other hand, if the measurement is too high, it will use too much energy and cause the system to wear down earlier than expected.
To fulfill the local exhaust airflow requirements of ASHRAE 62.2 (2010) and the International Residential Code (IRC 2021, Section M1507), bathroom fans should have a mechanical exhaust capacity of ≥ 50 cfm for intermittent operation or ≥ 20 cfm of ventilation when operated continuously.
Over-Sized Exhaust Fans
If an exhaust fan is too powerful for the space, it will cause drafts and make you feel cold (not great when you get out of a warm shower!).
How far can you run ducting for an exhaust fan in the bathroom? There is no guidance on the duct length; only a straight run is preferred to a duct with bends or elbows. On average, the vent duct can run from 2 to 30 feet.