Free Tool
Free HVAC duct size calculator
Enter your airflow and target velocity to get the round duct diameter and a rectangular equivalent. A quick sizing check for a single run. Free, no signup.
From the CFM calculator or the design
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Size a run by the air it moves
Duct size comes down to how much air you are moving and how fast you want it to travel. This uses the velocity method: the cross-section area is the airflow times 144, divided by the velocity you pick, and the round diameter follows from the area.
Velocity is the trade-off. A lower velocity is quieter and moves air with less resistance, but it needs a bigger duct. Residential supply runs are usually sized around 700 feet per minute, branches and returns a little slower, and a main trunk faster. Pick the target and the calculator sizes to it.
The round size rounds up to the next standard, and the rectangular equivalent gives you a size for a joist bay or a tight ceiling. Both move the same air at the velocity you set.
This is a starting point for one straight run, not a whole-house design. A proper Manual D lays out the entire system, accounts for every fitting and the static pressure of the equipment, and balances the branches, which is work for an HVAC pro.
Duct Size Calculator questions
How do I size a duct?
Divide the airflow by the velocity you want, adjust the units, and solve for the area, then the diameter. This calculator does it and rounds to the next standard size.
What velocity should I use?
Around 700 feet per minute for a residential supply, slower for branches and returns, faster for a main trunk. The calculator lets you pick.
Does this replace a Manual D?
No. This sizes a single run as a check. A Manual D designs the whole system, including fittings and static pressure, and belongs with a pro.
Is this really free?
Yes. No email, no signup, and nothing you type leaves your browser.
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Disclaimer: These free tools and templates are provided for general information and estimating purposes only. All results — including quantities, material and cost estimates, load, sizing, and span figures, scores, and any guidance or warnings — are rough approximations that may not fit your specific project, site conditions, local building codes, or current prices. They are not engineering, architectural, structural, legal, financial, or other professional advice, and are not a substitute for a licensed engineer, architect, contractor, or other qualified professional. Always confirm quantities, sizing, and structural, electrical, mechanical, and code requirements with a licensed professional and your local authority having jurisdiction before purchasing materials or performing any work. Werx makes no warranty as to accuracy and accepts no liability for any decision made using these tools. Use at your own risk. See our Terms of Service.