How to Calculate CFM for Bathroom Fan
Measure the dimensions of your bathroom., Calculate the floor area of your bathroom., Calculate the total volume of your bathroom., Determine the required CFM for your bathroom fan., Purchase a bathroom fan with the required CFM rating or higher.
Step-by-Step Guide
-
Step 1: Measure the dimensions of your bathroom.
Use a tape measure to measure each dimension of your bathroom
- the length of each wall and the height from floor to ceiling.
For a bathroom with a rectangular floor plan, you can keep these numbers in your head for the CFM calculation.
If you have an irregular floor plan, you may want to draw and label the plan on paper to help you calculate your bathroom's volume. -
Step 2: Calculate the floor area of your bathroom.
For a rectangular bathroom, this is done by multiplying the width and length.
For example, a 7 ft x 10 ft (2.1 m x 3 m) bathroom has a floor area of 70 square feet (6.5 square meters).
If your bathroom has a more complex shape, break the floor plan into simpler shapes, calculate their areas, and then add those areas together to arrive at the total floor area. Don't ignore the air above the bathtub--it is part of the total volume. , This is done by multiplying your bathroom's floor area by the ceiling height.
For example, a bathroom with a 90 square feet (8.4 square meters) floor area and a 9 ft (2.7 m) ceiling height has a total volume of 810 cubic feet (22.9 cubic meters).
Your bathroom ceiling may not have a uniform height; for example, it may be slanted or vaulted.
In this case, estimate the average ceiling height (somewhere in between the lowest and highest points) and use that in your calculation. , An effective bathroom fan should be capable of replacing the entire volume of air 8 times per hour, or every
7.5 minutes.
To arrive at your required CFM, simply divide your bathroom's volume by
7.5.
For example, a bathroom with a volume of 1000 cubic feet (28.3 cubic meters) needs a fan capable of moving (1000 /
7.5) or 133 CFM (3.8 cubic meters per minute). , After calculating your required CFM, make sure to check the rating of any fan your purchase.
A fan with too low of a rating won't remove moisture effectively, while buying a fan with too much power may lead to unnecessary noise (as more powerful fans tend to be louder).
Aim for a CFM rating just above your requirement. -
Step 3: Calculate the total volume of your bathroom.
-
Step 4: Determine the required CFM for your bathroom fan.
-
Step 5: Purchase a bathroom fan with the required CFM rating or higher.
Detailed Guide
Use a tape measure to measure each dimension of your bathroom
- the length of each wall and the height from floor to ceiling.
For a bathroom with a rectangular floor plan, you can keep these numbers in your head for the CFM calculation.
If you have an irregular floor plan, you may want to draw and label the plan on paper to help you calculate your bathroom's volume.
For a rectangular bathroom, this is done by multiplying the width and length.
For example, a 7 ft x 10 ft (2.1 m x 3 m) bathroom has a floor area of 70 square feet (6.5 square meters).
If your bathroom has a more complex shape, break the floor plan into simpler shapes, calculate their areas, and then add those areas together to arrive at the total floor area. Don't ignore the air above the bathtub--it is part of the total volume. , This is done by multiplying your bathroom's floor area by the ceiling height.
For example, a bathroom with a 90 square feet (8.4 square meters) floor area and a 9 ft (2.7 m) ceiling height has a total volume of 810 cubic feet (22.9 cubic meters).
Your bathroom ceiling may not have a uniform height; for example, it may be slanted or vaulted.
In this case, estimate the average ceiling height (somewhere in between the lowest and highest points) and use that in your calculation. , An effective bathroom fan should be capable of replacing the entire volume of air 8 times per hour, or every
7.5 minutes.
To arrive at your required CFM, simply divide your bathroom's volume by
7.5.
For example, a bathroom with a volume of 1000 cubic feet (28.3 cubic meters) needs a fan capable of moving (1000 /
7.5) or 133 CFM (3.8 cubic meters per minute). , After calculating your required CFM, make sure to check the rating of any fan your purchase.
A fan with too low of a rating won't remove moisture effectively, while buying a fan with too much power may lead to unnecessary noise (as more powerful fans tend to be louder).
Aim for a CFM rating just above your requirement.
About the Author
Charles Young
Writer and educator with a focus on practical DIY projects knowledge.
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