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How to Calculate CFM for Ducts

Air flow, usually calculated in cubic feet per minute (CFM), can be calculated by the area of a duct's cross section and the velocity of the air itself. You may be able to reference the velocity, but if not, a device may be required to measure it for you. These devices measure velocity based on pressure. Assuming you can acquire the necessary data, calculating the CFM is relatively painless.

Things You'll Need

  • Tape measure
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Instructions

  1. Square Ducts

    • 1

      Measure two consecutive sides of the duct, in inches.

    • 2

      Convert inches into feet by dividing each measurement by 12.

    • 3

      Calculate the cross-sectional area of the duct by multiplying the two numbers together.

    • 4

      Multiply the cross-sectional area by the air velocity, measured in feet per minute, to calculate air flow in CFM.

    Round Ducts

    • 5

      Measure the diameter of the opening.

    • 6

      Convert the measurement into inches by dividing it by 12.

    • 7

      Divide the diameter by two to calculate the radius.

    • 8

      Square the radius and multiple it by PI, or 3.14, to calculate the area.

    • 9

      Multiply the area by the air velocity, measured in feet per minute, to calculate air flow in CFM.

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