HVAC
Duct Size Calculator
Size duct from airflow and friction rate as a planning check, alongside a proper Manual D rather than in place of it.
Result
This is velocity-based sizing, which is a first approximation. Proper duct design (ACCA Manual D) works from a total friction rate across the whole system including fittings, and equivalent rectangular sizing is not simply equal area. It carries a friction penalty.
Get a Free Business AuditWhat moves the number
- Available static pressure, which sets the friction rate the design can use.
- Duct material, since flex, rigid and lined duct have very different resistance.
- Fittings, which frequently contribute more resistance than the straight runs.
Where this one goes wrong
Sizing straight duct correctly and ignoring the fittings. Elbows, takeoffs and transitions carry equivalent length that can exceed the actual run, and a design that counts only the straight sections comes out undersized.
Before you quote
- Total equivalent length including every fitting on the critical path.
- Whether the return path is sized as carefully as the supply.
- Whether flex duct runs are pulled tight, since sag multiplies resistance.
Questions
About this calculator
Does this replace Manual D?
No. It is a planning check for a single run. A full design distributes the available static pressure across the whole system and balances the branches, which is what Manual D is for.
Why does flex duct need different sizing?
Because its inner liner is corrugated and it is rarely installed perfectly straight. Compressed or sagging flex has dramatically higher resistance than its nominal rating suggests.
What friction rate should I design to?
Whatever the available static pressure and the total equivalent length allow, which is a calculation rather than a constant. Using a default figure on a long duct run is how systems end up starved.
The formula
duct area = CFM ÷ velocity; round diameter = √(4 × area ÷ π)
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How to use this calculator
- Airflow (CFM). Enter your own figure; the page starts at 400.
- Design velocity (feet per minute). 600–900 for branches, 900–1,200 for trunks.
- Rectangular duct height (inches) (in). Enter your own figure; the page starts at 8 in.
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 |
|---|---|
| Airflow (CFM) | 400 |
| Design velocity (feet per minute) | 700 |
| Rectangular duct height (inches) | 8 in |
What comes out
| Result | Value |
|---|---|
| Round duct diameter main result | 10.2 |
| Nearest standard round size | 12" round |
| Rectangular width at that height | 10.3 |
| Duct cross-section | 82.3 |
400 CFM at 700 fpm needs 10.2" round. Use 12" round, or 10.3" × 8" rectangular.
How sensitive the answer is
Holding everything else steady and changing only airflow (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.
| Airflow (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
- Airflow (CFM)
- A number, between 0 and 100000, starting at 400.
- Design velocity (feet per minute)
- 600–900 for branches, 900–1,200 for trunks.
- Rectangular duct height (inches) (in)
- Measured in in, between 0 and 60, starting at 8 in.
What the results tell you
| Result | What it is |
|---|---|
| Round duct diameter | The headline result. A decimal figure. |
| Nearest standard round size | A category or planning selection, rather than a measured quantity. |
| Rectangular width at that height | A decimal figure. |
| Duct cross-section | A decimal figure. |
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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