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How to Calculate Wattage for a Bathroom Ventilation Fan

Mikhail Polenin

To be effective, a bathroom fan should be able to replace the air in the room eight time per hour. Ventilation fans are labeled based on the amount of air they can exchange per minute for each cubic foot of space, or CFM. You can compare how efficient fans are by dividing their CFMs by the number of watts they use. The more efficient the fan, the higher the CFM/Watt number. In most cases manufacturers provide both figures, on older fans it might take a bit of detective work to calculate the wattage from the other data that the manufacturer provides.

Bathroom fans help prevent mold and mildew.

Step 1

Look around the fan’s edge for printed data from the manufacturer. You usually find this around the certification emblems on the casing.

Step 2

Look for a number followed by “V” and one followed by “A.” These numbers represent the voltage and amperage of the fan. If you don’t find a number followed by a “V,” assume the number is 115 or 220, depending on the region of the world you live in. If you live in America, you use a 115 volt circuit. In Europe, you often use 220 volts.

Step 3

Multiply the two numbers. The result is the wattage of your fan. If you have a fan that reads 115V and 1.5A, then the fan uses 172.5 watts.