If you have a round duct, you can use basic math to calculate the area of the duct. The area of the duct is how much room two dimensions of the duct take up. It is not volume of the duct which measures in three dimensions. The formula for the area of a circle is Pi times radius squared. Pi is estimated at 3.14, however if you want to find a more precise calculation, then you can take pi out to further decimal places.
Instructions
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1
Measure the duct's diameter. For example, assume your diameter is 2 feet.
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2
Divide your diameter by 2 to calculate your radius. In the example, 2 feet divided by 2 equals 1 foot.
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3
Multiply your radius by itself. This is squaring your radius. In the example, 1 times 1 equals 1 foot squared.
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Multiply your squared radius by pi. In the example, 1 foot squared times 3.14 equals 3.14 feet squared.