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.

- Skill level:
- Easy

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### Things you need

- Tape measure

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## Instructions

- 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.

- 1
Measure the diameter of the opening.

- 2
Convert the measurement into inches by dividing it by 12.

- 3
Divide the diameter by two to calculate the radius.

- 4
Square the radius and multiple it by PI, or 3.14, to calculate the area.

- 5
Multiply the area by the air velocity, measured in feet per minute, to calculate air flow in CFM.