Duct CFM to Diameter Reference Values
Estimate round duct diameter from airflow (CFM) and target air velocity (FPM).
On this page
Reference Value Index
This page lists 36 reference values for quick browsing and lookup.
All Reference Values
- 100 CFM at 500 FPM = 6.06 in round duct
- 100 CFM at 700 FPM = 5.12 in round duct
- 100 CFM at 900 FPM = 4.51 in round duct
- 100 CFM at 1100 FPM = 4.08 in round duct
- 150 CFM at 500 FPM = 7.42 in round duct
- 150 CFM at 700 FPM = 6.27 in round duct
- 150 CFM at 900 FPM = 5.53 in round duct
- 150 CFM at 1100 FPM = 5 in round duct
- 200 CFM at 500 FPM = 8.56 in round duct
- 200 CFM at 700 FPM = 7.24 in round duct
- 200 CFM at 900 FPM = 6.38 in round duct
- 200 CFM at 1100 FPM = 5.77 in round duct
- 300 CFM at 500 FPM = 10.49 in round duct
- 300 CFM at 700 FPM = 8.86 in round duct
- 300 CFM at 900 FPM = 7.82 in round duct
- 300 CFM at 1100 FPM = 7.07 in round duct
- 400 CFM at 500 FPM = 12.11 in round duct
- 400 CFM at 700 FPM = 10.24 in round duct
- 400 CFM at 900 FPM = 9.03 in round duct
- 400 CFM at 1100 FPM = 8.17 in round duct
- 600 CFM at 500 FPM = 14.83 in round duct
- 600 CFM at 700 FPM = 12.54 in round duct
- 600 CFM at 900 FPM = 11.06 in round duct
- 600 CFM at 1100 FPM = 10 in round duct
- 800 CFM at 500 FPM = 17.13 in round duct
- 800 CFM at 700 FPM = 14.48 in round duct
- 800 CFM at 900 FPM = 12.77 in round duct
- 800 CFM at 1100 FPM = 11.55 in round duct
- 1000 CFM at 500 FPM = 19.15 in round duct
- 1000 CFM at 700 FPM = 16.18 in round duct
- 1000 CFM at 900 FPM = 14.27 in round duct
- 1000 CFM at 1100 FPM = 12.91 in round duct
- 1200 CFM at 500 FPM = 20.98 in round duct
- 1200 CFM at 700 FPM = 17.73 in round duct
- 1200 CFM at 900 FPM = 15.64 in round duct
- 1200 CFM at 1100 FPM = 14.14 in round duct
Reference Table
| Airflow Scenario | Calculated Diameter | Area (ft²) | Nominal (in) |
|---|---|---|---|
| 100 CFM at 500 FPM | 6.06 in round duct | 0.2 | 6 |
| 100 CFM at 700 FPM | 5.12 in round duct | 0.143 | 5 |
| 100 CFM at 900 FPM | 4.51 in round duct | 0.111 | 5 |
| 100 CFM at 1100 FPM | 4.08 in round duct | 0.091 | 4 |
| 150 CFM at 500 FPM | 7.42 in round duct | 0.3 | 7 |
| 150 CFM at 700 FPM | 6.27 in round duct | 0.214 | 6 |
| 150 CFM at 900 FPM | 5.53 in round duct | 0.167 | 6 |
| 150 CFM at 1100 FPM | 5 in round duct | 0.136 | 5 |
| 200 CFM at 500 FPM | 8.56 in round duct | 0.4 | 9 |
| 200 CFM at 700 FPM | 7.24 in round duct | 0.286 | 7 |
| 200 CFM at 900 FPM | 6.38 in round duct | 0.222 | 6 |
| 200 CFM at 1100 FPM | 5.77 in round duct | 0.182 | 6 |
| 300 CFM at 500 FPM | 10.49 in round duct | 0.6 | 10 |
| 300 CFM at 700 FPM | 8.86 in round duct | 0.429 | 9 |
| 300 CFM at 900 FPM | 7.82 in round duct | 0.333 | 8 |
| 300 CFM at 1100 FPM | 7.07 in round duct | 0.273 | 7 |
| 400 CFM at 500 FPM | 12.11 in round duct | 0.8 | 12 |
| 400 CFM at 700 FPM | 10.24 in round duct | 0.571 | 10 |
| 400 CFM at 900 FPM | 9.03 in round duct | 0.444 | 9 |
| 400 CFM at 1100 FPM | 8.17 in round duct | 0.364 | 8 |
| 600 CFM at 500 FPM | 14.83 in round duct | 1.2 | 15 |
| 600 CFM at 700 FPM | 12.54 in round duct | 0.857 | 13 |
| 600 CFM at 900 FPM | 11.06 in round duct | 0.667 | 11 |
| 600 CFM at 1100 FPM | 10 in round duct | 0.545 | 10 |
| 800 CFM at 500 FPM | 17.13 in round duct | 1.6 | 17 |
| 800 CFM at 700 FPM | 14.48 in round duct | 1.143 | 14 |
| 800 CFM at 900 FPM | 12.77 in round duct | 0.889 | 13 |
| 800 CFM at 1100 FPM | 11.55 in round duct | 0.727 | 12 |
| 1000 CFM at 500 FPM | 19.15 in round duct | 2 | 19 |
| 1000 CFM at 700 FPM | 16.18 in round duct | 1.429 | 16 |
| 1000 CFM at 900 FPM | 14.27 in round duct | 1.111 | 14 |
| 1000 CFM at 1100 FPM | 12.91 in round duct | 0.909 | 13 |
| 1200 CFM at 500 FPM | 20.98 in round duct | 2.4 | 21 |
| 1200 CFM at 700 FPM | 17.73 in round duct | 1.714 | 18 |
| 1200 CFM at 900 FPM | 15.64 in round duct | 1.333 | 16 |
| 1200 CFM at 1100 FPM | 14.14 in round duct | 1.091 | 14 |
Frequently Asked Questions
Frequently asked questions for Duct CFM to Diameter and its reference values.
How is round duct diameter estimated from CFM?
The calculator first finds area from CFM/velocity, then solves the round-duct area formula for diameter.
Why does velocity affect duct size?
Higher design velocity allows smaller ducts for the same airflow, while lower velocity requires larger ducts.
What velocity should I choose?
Choose based on noise, pressure drop, and system type. Main trunks often use lower velocity than small branch runs.
Is this enough for full duct design?
Use it for quick sizing; complete designs should include friction, fittings, static pressure, and equipment constraints.
Methodology and Review
Each row on this page links to a value detail page derived from the same conversion data file as the main calculator and reference table. This reduces inconsistencies across page variants.
Editorial metadata and structured data are rendered across the converter page and linked value pages. Review policies are documented in our review process and editorial policy.