The velocity of airflow is measured in feet per minute (FPM). An anemometer can give you this simple reading in seconds. Converting this measurement to reflect the volume of airflow, measured in cubic feet per minute (CFPM), requires measuring the area of the opening through which the air is flowing. Once you have this information at hand, the calculation is straightforward.
Things You'll Need
 Measuring tape
 Calculator

Use a measuring tape to get the height and width of a square or rectangular opening. For a round opening, determine the radius by dividing the diameter of the opening in half.

Determine the area of your opening. To find the area of a square or rectangular opening, multiply height by width. A 2 by 4 foot opening would be: 2 x 4 = 8 square feet. For round openings, square the radius of the opening and multiply the square by pi. To calculate a 4foot round opening: 2 x 2 = 4 x 3.1415 = 12.566 square feet.

Multiply the FPM by the area of the opening. With a velocity of 20 FPM and an area of 12.566, the flow volume would be: 20 x 12.566 = 251.32 CFPM.
Tips & Warnings
 For openings with an area of less than 1 foot, divide the number of square inches in the area by 144, the number of inches in a cubic foot, to determine the CFPM. A pipe with a 9inch opening would be: 3 x 3 = 9 x 3.1415 = 28.2735 square inches. Divide this number by 144 to get 0.1963 square feet.
References
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