Duct Size Calculator

Frequently Asked Questions

How is round duct size calculated from CFM?

Area (sq ft) = CFM ÷ velocity (fpm), then diameter (in) = √(area × 4 ÷ π) × 12. 400 CFM supply at 700 fpm needs 400 ÷ 700 = 0.571 sq ft → ≈ 10.2 in round, so use 10 in.

What target velocities should I use?

ASHRAE residential rules of thumb: supply trunks 700–900 fpm, returns 600–700 fpm, branch runs 600 fpm, registers ≤ 500 fpm. Higher velocities = smaller ducts but more noise and static pressure.

How do I convert round to rectangular duct?

Use the equivalent-diameter formula De = 1.30 × (a·b)^0.625 ÷ (a+b)^0.25. A 10 in round equals roughly an 8 × 12 in rectangular or a 7 × 14 in - pick the size that fits the joist bay.

Why does aspect ratio matter for rectangular duct?

Aspect ratios above 4:1 waste sheet metal and add friction. Keep rectangular duct closer to 2:1 or 3:1 (e.g. 8×16, not 4×30) for efficient airflow per dollar of sheet metal.

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How This Calculator Works

HVAC ducts must be sized so that air moves fast enough to actually reach the register, but slowly enough that the system stays quiet and the blower does not stall against high static pressure. Industry practice settles on roughly 700 fpm in supply branches and 600 fpm in returns for residential work; main trunks run a touch faster, low-velocity systems a touch slower. From those targets the cross-sectional area follows directly - area (sq ft) = CFM ÷ velocity - and from the area you can solve for either a round-duct diameter or a rectangular height given a chosen width. This tool returns the required round diameter, the next even nominal size up, and the equivalent rectangular duct using the ASHRAE equivalent-diameter relation.

Formula / Method

Required area A (sq ft) = CFM ÷ velocity (fpm). Convert to square inches by ×144. Round-duct diameter d = 2√(A ÷ π) inches. The ASHRAE equivalent-diameter formula for a rectangular duct of sides a and b that flows the same as a round duct of diameter D_e is D_e = 1.30 × (a·b)^0.625 ÷ (a + b)^0.25. Given a chosen width, the calculator iterates the height until D_e matches the round diameter you need, then rounds up to a stock size. Always size up to the next available duct - you can throttle airflow with a damper, but you cannot push more air through pipe that is too small without raising velocity, noise, and static pressure.

Worked Example

A bedroom needs 120 CFM of supply air at the standard 700 fpm branch velocity. Area = 120 ÷ 700 ≈ 0.171 sq ft = 24.7 sq in. Round diameter = 2√(24.7 ÷ π) ≈ 5.6 in, so install a 6 in flex or rigid round. As an equivalent rectangular duct 8 in wide, solving D_e = 5.6 gives roughly 4 in tall - a 4 × 8 rectangular trunk-stub flows the same air as a 6 in round. Scale up: a 400 CFM return at 600 fpm needs 96 sq in, an 11 in round (round up to 12 in), or roughly 8 × 14 rectangular.

Common Mistakes

Sizing returns at supply velocity - returns run slower, so they need more cross-section for the same CFM.

Forgetting that flex duct loses about 30% effective capacity per equivalent foot of stretch and every bend; size up one nominal step when long flex runs are unavoidable.

Using nominal "6 in" for tightly compressed flex that is really flowing like a 4 in pipe.

Sizing the trunk for the sum of all branches at branch velocity - trunks run at higher velocity (800–1000 fpm) so they can be smaller than the simple sum.

Ignoring the equivalent length penalty of elbows and tees - an unsupported 90° flex elbow can equal 20–30 ft of straight duct.

Tips & FAQ

What velocity should I use? 700 fpm supply branches and 600 fpm returns are quiet-system defaults for homes; commercial low-velocity systems run 1000–1500 fpm in trunks. Above ~900 fpm at registers you start to hear the air. Round vs. rectangular? Round duct has lower friction loss per CFM and is cheaper for the airflow it carries; use rectangular only where ceiling depth forces it (joist bays, soffits). What is "equivalent diameter"? The round duct that has the same friction loss per foot as your rectangular duct at the same airflow - it is what you compare to friction-loss charts. Should I just size every branch to 6 in? Tempting but wasteful - tiny rooms over-supplied become drafty and noisy, large rooms under-supplied stay uncomfortable. How do I check static pressure? Pair this tool with the Static Pressure (TESP) calculator - a well-sized duct system stays under 0.5 in w.c. on a residential blower. Sealing matters more than size: a properly sized but leaky duct system can lose 20–30% of conditioned air into the attic; mastic every joint. Insulate exposed runs in unconditioned space (R-6 minimum, R-8 in hot climates). These are planning estimates; for permitted work follow ACCA Manual D and verify with a balancing hood after install.

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