About

DuctSizingCalculator.com is an independent HVAC duct-sizing tool built for the specific problem of turning an airflow number into an actual duct size — quickly, using the same engineering method a duct-sizing worksheet or ductulator is built on, without an account, a spreadsheet, or a paper wheel.

What the calculator actually does

Enter an airflow in CFM and either a target friction rate or a target velocity, and the tool solves the standard round-duct friction equation for galvanized sheet metal (FR = 0.109136 × Q1.9 / d5.02), rounds up to the nearest standard duct size, and reports the resulting velocity and actual friction rate — plus a set of equivalent rectangular duct sizes using ASHRAE's round-to-rectangular equivalent-diameter formula. This is the equal-friction method, the sizing approach used in ACCA Manual D and by most residential and light-commercial duct designers. Full mechanics, including every formula used, are on the How This Calculator Works page.

Who this is for

HVAC technicians and installers doing a quick field size check, homeowners and DIYers planning a duct run or trying to understand why a vent is noisy, and anyone sanity-checking a duct size against a quote or an existing installation. It's built for sizing individual duct runs quickly — not as a replacement for a full Manual D/Manual J system design, which accounts for room-by-room loads, total effective length with fitting equivalents, and whole-system static pressure budgets that a single-run calculator like this one doesn't attempt to model.

What it assumes, plainly stated

• Round duct sizing is based on smooth galvanized sheet metal — the standard reference material for the friction equation. Flex duct has meaningfully higher friction — commonly 3–8× depending on installation — so a flex run should be sized up from the metal-duct result.
• Friction rate is calculated for straight duct — it doesn't add equivalent-length penalties for elbows, transitions, or fittings.
• The same math applies to supply and return ducts; only the target velocity or friction rate you should aim for typically differs (returns are conventionally sized to a lower velocity, often 500–600 fpm).

Independence

This site is not affiliated with, endorsed by, or produced in partnership with ACCA, ASHRAE, SMACNA, or any duct, HVAC equipment, or sheet-metal manufacturer. The formulas used are standard, published engineering references, not proprietary calculations — the goal is a fast, accurate tool built on the same math the industry already uses, not a different or unofficial method.

How your data is handled

Every calculation runs entirely in your browser — the airflow and duct numbers you enter are never sent to or stored on our servers. See the full privacy policy.

Contact

Questions, corrections, or feedback: [email protected] — see the contact page for details.

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