DuctCalc Recommended duct velocity: the chart, and checking a real duct
For residential supply mains the app uses 700 to 900 fpm with a 1,000 fpm ceiling; branches and returns sit lower. Type the airflow and the duct you have and a coloured badge says whether the speed is in range.
What velocities does DuctCalc use?
There are six applications. Each has a recommended band, which you design inside for quiet, and a maximum, which you do not exceed on purpose.
| Application | Recommended | Maximum |
|---|---|---|
| Residential · supply-air main | 700-900 | 1,000 |
| Residential · supply-air branch | 500-700 | 900 |
| Residential · return air | 500-700 | 800 |
| Commercial · supply-air main | 1,200-1,500 | 2,000 |
| Commercial · supply-air branch | 800-1,000 | 1,300 |
| Commercial · return air | 800-1,000 | 1,500 |
The same table sits offline under Reference > Recommended air velocities, in m/s if the app is set to metric. The footer there tells you to treat the maximum as a hard ceiling.
How do you check a duct you already have?
- Open Check a duct and tap Round duct (or Rectangular duct).
- Enter the Airflow and the Diameter, or the Width and Height.
- Read Air velocity and the badge underneath.
- Tap the line under the badge to change the Application. It is the same choice as Settings > Check velocity against.
One duct, four verdicts
Take a 6 in galvanized round and judge it as a Residential · supply-air branch (500 to 700 fpm, maximum 900):
| Airflow | Velocity | Badge |
|---|---|---|
| 100 CFM | 509 fpm | Within recommended range |
| 150 CFM | 764 fpm | Above recommended band · check noise |
| 200 CFM | 1,019 fpm | Over maximum · resize |
The fourth state is Below range · quiet, duct oversized, which you get with the same branch band if you run 400 CFM through a 14 in duct (374 fpm). It is shown in grey, not amber: a big duct is not a fault, it is just more sheet metal than you needed.
What the badge does not tell you
It compares velocity with a table. It does not measure noise, and there is no application for a bedroom or a studio. A grille, a damper or an elbow right after the duct makes more noise than the straight run, and none of that is in the calculation.
Questions and answers
What is a good duct velocity for a house?
Does high velocity always mean noise?
Can I set my own velocity limits?
What is the velocity of a rectangular duct in the app?
More DuctCalc guides
- How to calculate duct size from CFM
- Round to rectangular duct equivalent, with real velocities
- Flex duct friction loss compared with galvanized and duct board
- CFM per ton: the chart, and the duct size that carries it
- How many CFM can a 6 inch duct carry?
- What is friction rate in ductwork, and what does a ductulator do with it?
- All questions about DuctCalc
- Back to DuctCalc