Engineering
Use this duct size calculator to calculate estimate round duct diameter from airflow and target air velocity.
Calculator
Choose what you want to calculate, enter realistic values, adjust units where shown, and use the scenario section for repeated-use planning.
This calculator applies the formula Area = CFM / velocity; diameter = sqrt(4A/pi). Enter each value with the matching unit, and AZCalculate updates the result instantly.
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Trust note
Use this result as a practical estimate. Real-world conditions, local rules, measurement tolerance, and changing inputs can affect the final outcome.
Area = CFM / velocity; diameter = sqrt(4A/pi)
This calculator applies the formula Area = CFM / velocity; diameter = sqrt(4A/pi). Enter each value with the matching unit, and AZCalculate updates the result instantly.
Assumptions
Avoid mistakes
Follow the reasoning, not only the final number.
Enter the known values in the calculator fields.
Starting with clearly defined values and units prevents the most common calculation errors.
Keep the units consistent with the labels shown beside each input.
This step transforms the known values into the form required by the formula.
Apply the formula: Area = CFM / velocity; diameter = sqrt(4A/pi).
This step transforms the known values into the form required by the formula.
Review the primary result and supporting values.
Compare the result with your real-world goal, such as size ducts.
Worked example
Duct area: 0.67 sq ft; Round duct diameter: 11.06 in
Calculator guide
Estimate round duct diameter from airflow and target air velocity. This page includes an interactive calculator, concise formula notes, worked examples, FAQs, related calculators, and practical guidance you can revisit whenever needed.
References
We work hard to keep every calculator accurate and useful. If you notice a calculation error, missing option, or unclear explanation, please let us know so we can review and correct it promptly.
Usage information loads after the calculator is ready.
FAQ
It uses Area = CFM / velocity; diameter = sqrt(4A/pi) and calculates the result from the values you enter.
Yes. AZCalculate calculator pages include copy, share, and print actions.
Yes for real engineering decisions. Use applicable standards and professional review.
Use a consistent unit system and convert all inputs before combining them.
Use it as an estimate and compare it with the formula, assumptions, and examples shown on the page.
Real-world inputs can include local rules, changing rates, measurement tolerances, and conditions outside the core formula.
Enter the known values requested by the calculator. Use the unit selectors where available and make sure the values describe the same scenario.
The calculator follows the formula shown on this page. Accuracy depends on the values, units, assumptions, and rounding used in your scenario.
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