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.