HVAC
HVAC Duct Size Calculator
Size a duct run from required airflow and the static pressure the system has available.
Result
Velocity sizing only. It does not account for friction loss along the run, fittings, or the total static pressure the blower can work against, all of which a proper duct design (Manual D) covers. Equivalent rectangular ducts carry slightly less than the same area in round.
Get a Free Business AuditWhat moves the number
- Available static pressure after the equipment, filter and coil have taken their share.
- Equivalent length of every fitting on the run.
- Whether the run is rigid, lined or flexible.
Where this one goes wrong
Designing every branch to the same friction rate as the shortest run. The critical path is the longest total equivalent length, and that is what the friction rate has to be based on.
Questions
About this calculator
What is the critical path?
The supply and return run with the greatest total equivalent length. It is the one that determines the friction rate available to the whole design, because if it works everything shorter will.
Can I just make the ducts bigger?
Oversized duct lowers velocity, which can cause poor throw at the register and stratification. Bigger is not automatically better; the design targets a velocity range as well as a pressure drop.
How much does a sharp elbow cost?
In equivalent length, a great deal, and several of them can exceed the straight run. That is why fittings belong in the calculation rather than being treated as incidental.
The formula
duct area = CFM / velocity; round diameter = sqrt(4 x area / pi)
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How to use this calculator
- Air flow (CFM). Enter your own figure; the page starts at 400.
- Where the duct runs. Enter your own figure; the page starts at Branch run (700 FPM).
- Duct shape. Enter your own figure; the page starts at Round.
The result updates as you type, so you can try a figure, see what it does to round duct diameter, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Air flow (CFM) | 400 |
| Where the duct runs | Branch run (700 FPM) |
| Duct shape | Round |
What comes out
| Result | Value |
|---|---|
| Round duct diameter main result | 10.2 |
| Nearest stocked size | 12 |
| Rectangular width needed | 0.0 |
| Nearest even rectangular width | 0 |
| Design velocity | 700 |
| Equivalent round diameter (in) | 0.0 |
| Aspect ratio | 0.0 |
| Velocity at the chosen size (FPM) | 509 |
Carrying 400 CFM needs about 10.2 inches round, so use the next size up: 12 inch.
How sensitive the answer is
Holding everything else steady and changing only air flow (cfm), this is what happens to round duct diameter. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Air flow (CFM) | Round duct diameter |
|---|---|
| 200 | 7.2 |
| 300 | 8.9 |
| 400 example above | 10.2 |
| 600 | 12.5 |
| 800 | 14.5 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Air flow (CFM)
- A number, between 0 and 40000, starting at 400.
- Where the duct runs
- Choose from Main trunk (900 FPM), Branch run (700 FPM), Return (600 FPM), Bedroom, keep it quiet (500 FPM).
- Duct shape
- Choose from Round, Rectangular.
What the results tell you
| Result | What it is |
|---|---|
| Round duct diameter | The headline result. A decimal figure. |
| Nearest stocked size | A number. |
| Rectangular width needed | A decimal figure. |
| Nearest even rectangular width | A number. |
| Design velocity | A number. |
| Equivalent round diameter (in) | A decimal figure. |
| Aspect ratio | A decimal figure. |
| Velocity at the chosen size (FPM) | A number. |
Before you rely on this number
Sizing, capacity and load figures here are planning estimates. A real installation depends on the manufacturer's data for the specific equipment, the duct system it is going into, and a proper load calculation for the building. Use this to sanity-check a number or price a job, not to select equipment.
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