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HVAC Duct Size Calculator

Find the recommended round duct diameter for a given airflow (CFM) and target air velocity.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-25 · v1.0.0

Market
Imperial · sales tax

Recommended round duct diameter

10.24 in (round duct diameter)

Check your inputs

This sizes for velocity only — a full duct design also accounts for static pressure and total system friction loss (duct length, fittings, filters) using manufacturer duct-sizing charts.

Required cross-sectional area
0.57 sq ft

With the figures above, the recommended round duct diameter comes to 10.2 in (round duct diameter). The method behind this is well established, though site conditions and material batches will move it somewhat. This is calculated for United States and cites IRC 2024. Building in another market? Change the selector above so the units and code reference follow.

Add the equipment this sizes

This result is a specification — 10.236 in (round duct diameter) — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • Standard duct sizing formula: cross-sectional area (sq ft) = CFM / velocity (ft/min); round duct diameter = sqrt(4 x area / pi)
  • Common design velocity for residential supply ducts is roughly 700-900 ft/min; return ducts are often designed slightly lower

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Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Frequently asked questions

How do I convert this to a rectangular duct size?
Use a duct equivalent-diameter conversion chart or calculator to find a rectangular duct (e.g. for tight spaces above a ceiling) with the same effective cross-sectional airflow capacity as this round diameter.
Why does velocity choice matter beyond just fitting the duct?
Higher velocities let you use smaller, cheaper ducts but increase both noise and static pressure (which taxes the blower motor) — main trunk lines and long branch runs are often designed toward the lower end of the velocity range for this reason.
Does this replace a Manual D duct design?
No — this estimates one duct run's size from velocity alone. A full residential duct system design (ACCA Manual D) also balances total system static pressure, fitting losses, and blower capability across all runs together.