Calculate CFM for Bathroom Fan
A bathroom fan is an essential piece of hardware in any indoor bathroom, without a window. The fan pulls air out of the bathroom and allows fresh air to be drawn in; this keeps the moisture from the shower, bath, or sink from encouraging mold growth. To be effective, a bathroom fan needs to be powerful enough to completely replace the air in the room about 8 times every hour. The effectiveness of the fan will be rated in "CFM," or cubic feet per minute of air moved. Learning how to calculate CFM for a bathroom fan will help you purchase the right size fan for your space.
Steps
- 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.
- 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.
- Calculate the total volume of your bathroom. 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.
- Determine the required CFM for your bathroom fan. 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).
- Purchase a bathroom fan with the required CFM rating or higher. 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.
Tips
- The measurements you take do not need to be incredibly precise. As long as your numbers are roughly accurate, the fan you choose should be effective.
- Many building codes require a minimum fan size regardless of bathroom size. A common requirement is 50 CFM.
Things You'll Need
- Tape measure
- Pencil
- Paper
- Calculator
