Duct CFM to Diameter

Estimate round duct diameter from airflow (CFM) and target air velocity (FPM).

Author

Dr. Lars Eriksson

HVAC editorial contributor

Swedish environmental economist from Stockholm School of Economics, advisor to the EU on carbon pricing policy

Reviewed by

Dr. Pablo Gutierrez

HVAC content reviewer

Chilean climate scientist at the University of Chile, modeling glacier retreat in the Patagonian ice fields

Last updatedFebruary 22, 2026

PublishedFebruary 22, 2026

Reference Values

Browse 36 reference values with individual detail pages for quick lookup.

Browse all reference values

Duct CFM to Diameter Calculator

Estimate a round duct diameter from airflow and target velocity.

Estimated Round Diameter

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Reference Table

Use this complete table for quick lookup and internal linking to specific value pages.

Duct CFM to Diameter Reference
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

Common questions about Duct CFM to Diameter, formulas, and typical use cases.

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

This page combines a live calculator, precomputed reference values, and FAQ content from the same conversion definition to reduce mismatch between calculator output and lookup tables.

Editorial metadata (author, reviewer, and update date) is rendered on the page and in structured data. See our editorial policy, review process, and corrections policy.

For compliance-sensitive work, verify final values against project requirements, governing standards, and manufacturer documentation.