Duct Size Calculator
Enter your CFM airflow and duct shape to get the cross-section area and, for round duct, the diameter.
What your result means
This uses the rough rule of about 1 square inch of duct per CFM at 0.1 inch of friction. For round duct it also gives the equivalent diameter. It's a starting point only; real duct design is an ACCA Manual D job that accounts for the whole run, fittings, and static pressure, and code wants Manual J and D.
How to use this calculator
- Get CFM requirement from your HVAC unit specs.
- Choose round or rectangular duct.
- Read duct cross-section area in sq in.
- For round, get equivalent diameter.
- For full sizing, use ACCA Manual D.
The formula
The rough rule is one square inch of duct per CFM. For a round duct, the diameter comes from the circle-area formula solved for diameter.
Worked example
Say you need 400 CFM in a round duct. That's a 400 sq in cross-section and a 22.6 inch diameter, which you'd round up to a 24 inch duct.
Duct sizing notes
| Point | Note |
|---|---|
| Rule of thumb | ~1 sq in per CFM |
| Round vs rectangular | round moves air more efficiently |
| Returns | slightly larger than supply |
| Precise sizing | ACCA Manual D |
Tips & gotchas
- Rule of thumb: 1 sq in of duct per CFM at 0.1 inch of friction.
- This is rough; real sizing uses ACCA Manual D.
- A larger duct is quieter and moves air more efficiently.
- Round ducts are more efficient than rectangular of the same area.
- Size the duct for the supply airflow, not the room.
Frequently asked questions
How big should my ducts be?
Rule of thumb is 1 sq in per CFM. Use ACCA Manual D for precision.
Round or rectangular ducts?
Round is more efficient; rectangular fits in walls better.
Why does duct sizing matter?
The wrong size is noisy, inefficient, and heats or cools unevenly.
Can I use this for return ducts?
Yes. Returns should be slightly larger than the supply.
Is this accurate enough for code?
For estimates only. Code wants ACCA Manual J and D calculations.
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Estimates only — see our full disclaimer.