How to Calculate Velocity of Air Through a Duct
If you have ever heard excessive noise coming from your home's ductwork or wondered why not very much air was coming out of the vents, the problem might be that the velocity of air coming through the ducts is too high. Too much air velocity can lead to excessive noise and highpressure air loss, actually diminishing the amount of air making it through. You can calculate the air velocity through your ducts in several ways, depending upon the other values with which you start.

Divide the air flow in in cubic feet per minute by the total area of the duct in square feet to get the air velocity in feet per minute.

Square the diameter of the duct in inches by multiplying it by itself. For example 10 squared equals 10 times 10, or 100. Multiply that result by Pi, which is approximately 3.14. Write the result down. Multiply the air flow in cubic feet per minute by 576. Divide the result by the number you wrote down to get the velocity in feet per minute.

Multiply the height of the duct in inches by the width of the duct in inches. In a square duct these two numbers will be the same. In a rectangular duct, they will be different. Write the number down. Multiply the air flow in cubic feet per minute by 144. Divide the result by the number you wrote down to get the air velocity in the duct in feet per minute.
Things You Will Need
 Calculator
 Air flow in cfm