Duct Velocity Calculator
Enter the CFM and the duct size. Get the velocity, the friction rate and the velocity pressure.
How to use ↓Result
Duct Chart · Duct Velocity Calculator · ductchart.com
Estimates only: they can differ from your chart or the manufacturer's data by 5% or more (flex by 2 times or more), so verify accuracy for your own application against the design documents, manufacturer data and local code.
How to use
- Enter the airflow in CFM.
- Choose round or rectangular and enter the size in inches.
- Pick the duct type. Friction depends on it.
- Press Calculate. Check the velocity note under the numbers.
Duct velocity, friction and velocity pressure
Velocity in fpm is the airflow in CFM divided by the cross-section area of the duct in square feet. For a rectangular duct the calculator uses the real area of the rectangle. The friction rate uses the Darcy-Weisbach equation with the Colebrook friction factor for standard air, and for a rectangle it is worked out at the equal-friction round diameter.
Velocity pressure is the pressure of the moving air, from density times (velocity ÷ 1,097) squared, in inches of water gauge, with standard air at 0.075 lb/ft³. It is the number you use to size fitting losses and to read a pitot traverse.
The calculator flags two cases in plain words. Above about 1,200 fpm, noise and friction rise quickly, and the fitting equivalent lengths used on this site are only approximate below that speed. Below about 400 fpm the duct may be bigger than it needs to be, which is a common rule of thumb rather than a limit.
If the velocity is not where you want it, go back to the duct size calculator and size by target velocity, or look it up on the duct size chart.
Example: 400 CFM in a 12 in. round duct moves 509 fpm, with 0.036 in. w.g. per 100 ft of friction. Push 1,000 CFM through a 10 in. round and the velocity is 1,833 fpm, so the page flags it.
FAQ
What velocity is too high?
It depends on the job. Above about 1,200 fpm noise and friction climb quickly, so this page flags it. Design documents and the room’s noise needs come first.
Why is the rectangular friction at the equivalent round?
Equal friction at equal airflow is how an equivalent round diameter is defined, so the friction rate of a rectangle is the friction of its equivalent round.
Does velocity change with duct type?
No. Velocity depends only on airflow and area. Friction changes with the roughness of the duct type.