In HVAC, it’s sometimes useful to calculate the velocity of air in the ducts. All central air conditioners, for example, require ductwork with the specified size of ducts and airflow (measured in CFM).
To calculate the speed of air in the ducts, we use a duct velocity calculation (you can find the calculator below).
Let’s first look at how to calculate the air velocity in the ducts. We have to use this air velocity formula in restricted spaces (such as ducts):
V (Air Velocity) = Q (Airflow) / A (Duct Cross-Section)
V represents the air velocity and is expressed in FPM (feet per minute). Q is airflow and is expressed in CFM (cubic feet per minute). A is the duct cross-section; basically, the area of the ducts. The bigger the ducts, the more airflow they can handle.
In short, air velocity in the ducts is calculated by dividing airflow by duct cross-section. Basically, we are converting CFM to air velocity (FPM).
Airflow is expressed as a simple number. Example: Air conditioner has a max. airflow of 600 CFM.
To adequately calculate the speed of air in ducts, we have to primarily calculate the cross-section of the duct. Shape-wise, ducts can be:
- Round. The area of round flex ducts is calculated as π*R2 where R is the radius of the duct. Example: 10-inch round duct has a radius of 5 inches. Its cross-section is calculated as 3.14*52 = 78.5 sq inches or 0.545 sq ft.
- Rectangular. The area of rectangular ducts is calculated as x*y where x and y and the length of the sides of the ducts. Example: 10×12-inch rectangular duct has a cross-section of 10*12 = 120 sq inches or 0.833 sq ft.
With this in mind, we can use these handy duct velocity calculators to accurately estimate how fast the air is moving in our ducts. Below you will find 2 calculators; 1st one for round ducts and the 2nd one for rectangular ducts:
Air Velocity Calculator For Round Ducts (1st Calc)
Example: Let’s say you have circular flex ducts with a 12-inch diameter. The airflow is 500 CFM. If you put both of these numbers in the duct velocity calculator above, you get a 636.94 FPM. That’s a little more than 7 mph (miles per hour).
Here is the full round duct air velocity chart for 500 CFM airflow:
Round Duct Air Velocity Chart (500 CFM Airflow)
| Round Duct Diameter (Inches): | Round Duct Air Velocity (Feet Per Minute): |
| 5 Inch Round Duct | 3668.79 FPM (Feet Per Minute) |
| 6 Inch Round Duct | 2547.77 FPM (Feet Per Minute) |
| 7 Inch Round Duct | 1871.83 FPM (Feet Per Minute) |
| 8 Inch Round Duct | 1433.12 FPM (Feet Per Minute) |
| 9 Inch Round Duct | 1132.34 FPM (Feet Per Minute) |
| 10 Inch Round Duct | 917.20 FPM (Feet Per Minute) |
| 12 Inch Round Duct | 636.94 FPM (Feet Per Minute) |
| 14 Inch Round Duct | 467.96 FPM (Feet Per Minute) |
| 16 Inch Round Duct | 358.28 FPM (Feet Per Minute) |
| 18 Inch Round Duct | 283.09 FPM (Feet Per Minute) |
| 20 Inch Round Duct | 229.30 FPM (Feet Per Minute) |
| 22 Inch Round Duct | 189.50 FPM (Feet Per Minute) |
| 24 Inch Round Duct | 159.24 FPM (Feet Per Minute) |
| 26 Inch Round Duct | 135.68 FPM (Feet Per Minute) |
| 28 Inch Round Duct | 116.99 FPM (Feet Per Minute) |
| 30 Inch Round Duct | 101.91 FPM (Feet Per Minute) |
Air Velocity Calculator For Rectangular Ducts (2nd Calc)
Example: Let’s say you have rectangular ducts with 12-inch height and 12-inch width. The airflow is 500 CFM. What is the air velocity in the ducts? If you input all these numbers in the calculator above, you get 500 FPM (that’s 5.7 mph).
You can use both of these calculators freely to help you design ductworks for central air conditioning systems, for example. If you need more help determining the airflow in the ducts, you can use these duct sizing charts for adequate ductwork sizing.
Dear Sir,
I am working on a project. Where I have only air pressure at continuous mode that is 1000 psi. The pipe size is 8-inch in diameter. What will be approximate airflow and what is the velocity ( approximate )of that compressed air? Will you reply to me?
I see your web page and I hope, you will reply to me.
Hello there, if you have a known pressure, you can use Bernoulli’s equation to calculate airflow through an 8-inch pipe. It’s a bit complicated but you can look into that.
A rectangular duct of length 25ft has air velocity of 60ft/min. Calculate the Cfm?
Hi Moses, to calculate CFMs, you would need the cross-section of the ducts; width and height. Examples would be 4×4, 4×8, 10×12 inch ducts. When you know that, you can calculate CFMs like this: CFM = Air Velocity × Cross-section. Get the units in order (cross-section is in inches, you transform it into feet), and you get the resulting CFMs.
how to calculate the branches Velocity of exhaust air duct, we plan to divert our 6 exhaust machine in our main duct? It is possible to maintain the 2.5 m/s Velocity?
Hi there, it all has to do with cross-sections. If the initial cross-section matches the end cross-section (summary of all the ducts), the velocity should stay the same. If the end cross-section increases, the air velocity will decrease, and visa versa.