Pocket HVAC
What is duct velocity?
Duct velocity is how fast air moves through a duct, in feet per minute (FPM) or metres per second. It's the airflow divided by the duct's inside area, so the same CFM moves faster through a smaller duct.
It's a quick check on duct size: air that's moving too fast is noisy and takes more static pressure to push, and air that's moving too slowly means the duct is bigger than it needs to be. To size a duct for the airflow you need, use the ductulator. It also works the other way in the field. Measure the average velocity across a duct with an anemometer or a pitot tube, multiply by the inside area, and you have the airflow in CFM. Switch the calculator to Airflow and it does that math for you.
How it's calculated
Round area = π × (D ÷ 2)² Rectangular area = W × H Velocity (FPM) = CFM ÷ area in ft² (in² ÷ 144 = ft²) Velocity (m/s) = airflow in m³/s ÷ area in m² Airflow (CFM) = FPM × area in ft²; airflow (L/s) = m/s × area in m² × 1000 Velocity pressure (in. w.c.) = (FPM ÷ 4005)² Velocity pressure (Pa) = 0.5 × 1.204 × (m/s)² Equivalent round = 1.30 × (W × H)^0.625 ÷ (W + H)^0.25
Sources
- ASHRAE Handbook: Fundamentals, Duct Design chapter (velocity pressure for standard air, equivalent round diameter)
- Huebscher, R.G. (1948), "Friction Equivalents for Round, Square and Rectangular Ducts", ASHVE Transactions 54
- TSI Application Note TSI-127, Flow Rate Calculations Using an Anemometer (Rev. C, 2021): airflow from average velocity and duct area
If a number doesn't match your chart, email me what you entered and what you expected, and I'll check it against the source. You can also read how the numbers are checked.
Checked by Paul Williams · Updated
Questions
How do you calculate duct velocity?
Divide the airflow by the inside area of the duct. In US units that's CFM divided by square feet, which gives feet per minute (FPM). For a rectangular duct, multiply the width by the height in inches and divide by 144 to get square feet.
How do you find CFM from duct velocity?
Multiply the average velocity in FPM by the duct's inside area in square feet. TSI's worked example: a 24 × 36 in duct is 6 ft², so an average of 1,000 FPM across it moves 6,000 CFM. Switch the calculator to Airflow and it does this for round or rectangular duct, in CFM or L/s.
Should I measure the inside or the outside of the duct?
The inside. Air only moves through the open space, so duct liner makes the real area smaller than the outside size. Flex duct is sold by its inside diameter.
What is the equivalent round size of a rectangular duct?
It's the round duct with the same friction loss as the rectangular one at the same airflow. It isn't the same area: a 12 × 8 duct has 96 in² inside, but its equivalent round is about 10.7 in, which has about 90 in². The calculator uses Huebscher's equation, the one in the ASHRAE Handbook.
What is velocity pressure?
It's the pressure caused by the air moving. A pitot tube reads total pressure, and total minus static pressure is velocity pressure. For standard air, velocity in FPM is 4005 times the square root of velocity pressure in inches of water column.
