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How many CFM do I need for my Bathroom Exhaust Fan?

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.

Aspire Bathroom Heater & Exhaust Fan with Tricolour 20W LED Light Why the Right Sized Bathroom Exhaust Fan is Important

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.

What Does CFM Mean?

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.

How to Size Your Bathroom Exhaust Fan

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.

Method One: Calculating Standard Bathroom Exhaust Fan Size

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.

quiet exhaust fans Method Two: Bathrooms with High Ceilings

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.

Method Three: Large Bathrooms

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


In this instance, the minimum CFM for this bathroom would 150CFM.

Linear 3 in 1 Bathroom Heater with Exhaust Fan and LED Lights

Method Four: Enclosed Spaces

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.

Martec’s Bathroom Exhaust Fan Calculator

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!