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- 1). Measure the duct's diameter. For example, assume your diameter is 2 feet.
- 2). Divide your diameter by 2 to calculate your radius. In the example, 2 feet divided by 2 equals 1 foot.
- 3). Multiply your radius by itself. This is squaring your radius. In the example, 1 times 1 equals 1 foot squared.
- 4). Multiply your squared radius by pi. In the example, 1 foot squared times 3.14 equals 3.14 feet squared.
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