Duct Size Calculator
This duct calculator sizes a round duct from the airflow in CFM and the air velocity. Enter both and the air duct calculator returns the round duct diameter in inches. It works as an hvac duct calculator for supply, return, and branch runs, and pairs with the duct sizing chart below so you can check the result against common airflow values.
Results
The Duct Size Calculator is part of our Ventilation collection. Browse more ventilation to size, convert, and check your work alongside this tool.
What the duct calculator shows
A duct has to carry a set airflow without the air moving so fast that it whistles or so slow that it drops its heat. This duct calculator balances the two by taking the airflow in CFM and the target velocity and returning the round duct diameter that fits. Whether you call it an air duct calculator, a ductwork calculator, an hvac duct calculator, or an hvac ductwork calculator, it solves the same sizing problem for any run.
How the ductwork calculator sizes a duct
The ductwork calculator first finds the cross section area the airflow needs, then turns that area into a round diameter:
Higher velocity packs the same CFM into a smaller duct but raises noise and pressure loss, while lower velocity needs a bigger duct. This is the duct size calculation formula behind every duct calculator cfm result, and it is why the duct sizing chart later on lists a diameter for each airflow.
Duct size calculator example
A run that carries 1000 CFM at 800 ft/min. The duct size calculator applies the formula:
So a 15 inch round duct fits 1000 CFM at that velocity. Change the airflow and the air duct size calculator returns a new diameter at once.
How to use the air duct calculator
Enter the airflow in CFM and the air velocity in feet per minute, and the air duct calculator returns the round duct diameter in inches along with the cross section area. Supply trunks run near 800 to 900 ft/min, branches lower, and returns lower still. Leave the velocity at 800 for a typical residential run, or lower it for a quieter duct. An air duct cfm calculator like this turns a target airflow straight into a size.
Reading the hvac duct calculator result
The round duct diameter is the size that carries your airflow at the velocity you chose. Round up to the next stock size, since a slightly larger duct is quieter and loses less pressure than a tight one. If the diameter comes out large for the space, raise the velocity a little and the hvac duct calculator returns a smaller duct, at the cost of some noise. To convert the round size to a rectangular duct, match the cross section area the tool reports.
When to use the air duct size calculator
- Sizing a supply trunk with the air duct size calculator
- Checking a branch run against a cfm chart for duct sizing
- Planning a return with a ductwork calculator
- Working as an air duct design calculator for a new system
Common air duct calculator mistakes
- Sizing every run at one velocity. Returns should run slower than supply trunks, so do not reuse a single number across the air duct calculator.
- Ignoring flex duct drag. A flex duct cfm chart runs a size larger than rigid metal for the same airflow.
- Not rounding up. Pick the next stock size above the diameter for lower noise and pressure loss.
Tips for sizing ducts
- Keep supply trunks near 800 to 900 ft/min and returns quieter.
- Round the diameter up to the next available duct size.
- Bump flexible runs one size over the rigid metal value.
How the ductwork calculator balances size and velocity
Every duct is a trade between size and speed. Push the air faster and the ductwork calculator returns a smaller duct that fits tight framing, but the faster air is louder and drags more pressure, so the fan works harder. Slow the air down and the duct grows, the system runs quiet, and pressure loss falls, but the duct may not fit the joist bay. Good design keeps supply trunks near 800 to 900 ft/min, branches around 600, and returns lower, which is why an air duct design calculator asks for velocity rather than fixing it. The fan power calculator shows how that velocity choice feeds into the fan energy the system needs.
Reading a cfm chart for duct sizing
A cfm chart for duct sizing lists a diameter for each airflow at a set velocity, and it is the fastest sanity check on any result. Read your CFM down the left, read the diameter across, and compare it with what the tool returns; a large gap usually means the velocity is off. Flexible duct needs a size bump over rigid metal because its ribbed wall drags more, so a flex duct cfm chart runs one size larger for the same airflow. The chart and this ductwork calculator agree because both use the same duct size calculation formula, only the chart fixes the velocity while the tool lets you set it.
Duct sizing chart and cfm duct chart
Round duct diameter for common airflows at 800 ft/min. Use it as a duct sizing chart, a duct chart for cfm, a cfm duct chart, or a ductwork cfm chart to size ductwork by CFM; it reads the same as any duct sizing chart cfm you would find in a manual.
| Airflow (CFM) | Round duct diameter |
|---|---|
| 100 CFM | 4.8 in |
| 200 CFM | 6.8 in |
| 400 CFM | 9.6 in |
| 600 CFM | 11.7 in |
| 1000 CFM | 15.1 in |
| 2000 CFM | 21.4 in |
Duct Size Calculator FAQ
How does the duct size calculator work?
It divides the CFM by the air velocity to get the cross section area, then converts that area to a round diameter. The duct calculator returns the diameter in inches.
What velocity should I use in an hvac duct calculator?
Around 800 to 900 ft/min for supply trunks, lower for branches, and lower still for returns. Lower velocity means a larger but quieter duct.
How do I read a duct sizing chart?
Find your airflow in the cfm duct chart and read across to the diameter. The chart above lists round sizes at 800 ft/min so you can size ductwork by CFM at a glance.
Can I size a rectangular duct with this?
Yes. Size the round duct first, then match its cross section area with a rectangular duct that carries the same airflow.