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Duct Size Calculator

Size a round or rectangular duct for a required airflow at a target velocity, with the noise threshold shown.

Computed in your browser — nothing you enter is uploaded. Figures are presented in United States units and terminology, and each formula’s basis is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax

Round duct diameter

10.2 in diameter

Check your inputs

Velocity sizing only. A complete design also balances the friction rate across the whole system so every branch delivers its share — see Manual D.

Duct area
82.29 in²
Diameter (mm)
259.99 mm
Rectangular, 6 in deep
15.42 in wide
Rectangular, 8 in deep
11.03 in wide
Airflow in L/s
400 CFM

At the values currently entered, the round duct diameter works out to 10.2 in diameter. Confidence is moderate: the method is sound, but real materials and site conditions vary. Figures are shown in United States units and terminology; switch the market above if you are building elsewhere.

Add the equipment this sizes

This result is a specification — 10.2 in 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 — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.

Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Regulatory standards & verification citations

  • ACCA Manual D — residential duct design; velocity and friction-rate method
  • ASHRAE Handbook — Fundamentals, Duct Design: recommended residential velocities of 3-5 m/s (600-900 fpm) for branches, up to 6-9 m/s (1200-1800 fpm) for trunks

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Frequently asked questions

Why does duct size matter so much?
Because airflow is what actually moves heat, and a duct too small for the flow raises velocity, noise and static pressure at once. High static pressure reduces the air the blower can deliver, which reduces capacity, which makes an correctly-sized unit behave like an undersized one. Most 'my system cannot keep up' complaints are duct problems, not equipment problems.
Round or rectangular?
Round moves air with less friction for the same cross-sectional area and is quieter, so use it wherever there is space. Rectangular exists because a joist bay is rectangular. The breakdown gives the equal-friction rectangular equivalents — note how much wider a shallow duct has to be, which is the cost of the flatter shape.
How much CFM does a room need?
Its share of the total, in proportion to its calculated load — not its floor area. A room with west-facing glass needs more air than a north-facing room of the same size. Splitting airflow by area is a common shortcut and it is why one room in so many houses is always too hot.
What if the calculated size is not available?
Go up, never down. Ducts come in fixed sizes; the next size up lowers velocity, noise and pressure loss slightly, all of which are improvements. The next size down does the opposite to all three at once.