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Duct Size Calculator

Size round and rectangular duct from airflow and friction rate, with velocity checks.

Airflow
CFM

Roughly 400 CFM per ton of cooling for the whole system, divided among the branches by room load.

In. w.c. per 100 ft of effective length. Manual D derives this from available static pressure divided by total effective length.

The duct
Material
Where it runs

The velocity ceiling is what keeps the duct from becoming audible.

Sizing a single duct is one step of a duct design. A full Manual D adds up the total effective length of every fitting on the worst run and compares it against the blower table before a friction rate is chosen at all.

Round duct size

10"round

Calculated requirement is 9.21" — rounded up to the next stocked size.
Velocity at this sizeSupply branch ceiling is 600 FPM
550 FPM
Actual friction at this sizeDesign rate was 0.08 in/100 ft
0.053 in/100 ft
Free area0.545 sq ft
78.5 sq in
Capacity at the design rate81% of this duct's capacity is used
372 CFM

Velocity is inside the quiet range for a supply branch

At 550 FPM this duct runs below the 600 FPM ceiling, so it should not be audible in a quiet room. Register and grille selection still matters — most duct noise complaints trace to an undersized register face, not the duct behind it.
Rectangular duct equivalent to 10" round
Rectangular sizeEquivalent roundFree areaVelocityAspect ratio
10" × 8"9.76"80 sq in540 FPM1.3:1
14" × 6"9.80"84 sq in514 FPM2.3:1
What each stocked round size carries at this friction rate
DiameterCapacityVelocity at capacityVelocity at your CFM
8"207 CFM592 FPM859 FPM
9"282 CFM638 FPM679 FPM
10" — selected372 CFM683 FPM550 FPM
12"603 CFM768 FPM382 FPM
14"906 CFM848 FPM281 FPM

Equivalent rectangular sizes come from the Huebscher relationship, De = 1.30 × (a × b)^0.625 ÷ (a + b)^0.25. Two ducts with the same equivalent round diameter move the same air at the same friction rate, but a flatter duct has more surface area, more heat loss and more sheet metal, so aspect ratios beyond about 4:1 are avoided where framing allows.

What this assumes

  • Based on the Darcy-Weisbach friction relationship for galvanized duct at standard air density.
  • Default friction rate is 0.08 in. w.c. per 100 feet, typical for residential systems.
  • Flexible duct carries significantly higher friction and should be sized one step larger than rigid.

Important

A full duct design requires ACCA Manual D, including total effective length, fitting equivalent lengths and available static pressure from the blower table.

Understanding the result

Velocity drives noise

Residential supply trunks are generally kept under 900 feet per minute and branch runouts under 600 FPM. Above those figures, the duct becomes audible. Commercial systems tolerate higher velocity because ambient noise is higher and duct is usually lined.

Return air is the usual failure point

Undersized returns are the most common duct defect in residential systems. Starved returns raise static pressure, cut airflow across the coil and can freeze an evaporator. Return duct should be sized at least as generously as supply.

Flexible duct is not equivalent to rigid

Flexible duct has two to three times the friction loss of smooth galvanized duct at the same diameter. A 6-inch flex run should be sized as if it were 5-inch rigid, and kept as short and straight as possible. Long flex runs with sag are the single most common cause of rooms that never get enough air.

Equivalent rectangular sizes

Round duct is more efficient than rectangular at the same airflow because it has less surface area and lower friction. A 10-inch round duct moves the same air as a 9×12 rectangular duct. When space forces a rectangular trunk, use the equivalent diameter — not the same nominal dimension — for friction calculations.

Common questions

What size duct for 1000 CFM?
At a friction rate of 0.08 inches w.c. per 100 feet, 1000 CFM requires approximately a 12-inch round duct or a 10×14 rectangular duct. Velocity at that size is about 1,275 FPM — acceptable for a trunk but too fast for a branch runout, which should stay under 600 FPM.
What size return duct for 3 ton AC?
A 3-ton system moves roughly 1,200 CFM total. The return should handle at least 1,200 CFM — typically a 14×20 rectangular return grille with a 14-inch round or 12×20 rectangular duct behind it. Undersized returns are the most common cause of frozen evaporator coils.
What size duct for 400 CFM?
400 CFM needs approximately an 8-inch round duct at 0.08 friction rate per 100 feet. That gives a velocity of about 920 FPM — acceptable for a supply trunk but at the upper limit for a branch. A 10-inch round duct at 400 CFM drops velocity to about 590 FPM, which is quieter.
How do you convert CFM to duct size?
Duct size depends on CFM and the allowable friction rate. At 0.08 in. w.c. per 100 feet: 200 CFM needs 6-inch round, 400 CFM needs 8-inch, 600 CFM needs 9-inch, 800 CFM needs 10-inch, and 1,000 CFM needs 12-inch. Higher friction rates allow smaller duct but increase blower energy use and noise.
What size duct for a 12x12 room?
A 12×12 foot room with 8-foot ceilings needs roughly 80 to 120 CFM depending on load. A 6-inch round supply runout at 100 CFM gives about 460 FPM velocity — within the quiet range for a bedroom. The room also needs a return path, either a dedicated return or a transfer grille to the hallway.
What is the maximum air velocity in residential duct?
Supply trunks are typically kept under 900 FPM and branch runouts under 600 FPM to limit noise. Returns can run slightly higher because they are in less occupied spaces. Above 1,000 FPM in a branch, occupants hear whistling and rattling at registers.
What size flex duct for 200 CFM?
Use 8-inch flexible duct for 200 CFM, not 6-inch. Flex duct has higher friction than rigid, so sizing one step larger than the rigid equivalent is standard practice. Keep the run under 25 feet, fully extended with no sag, and supported every 4 feet.
How big should a return air grille be?
A return grille should provide at least 2 square feet of free area per 400 CFM. A 3-ton system at 1,200 CFM needs roughly 6 square feet of free grille area — typically one 20×30 return or two 14×20 returns. The grille free area is less than its nominal size because of louvers and frame.
What friction rate should I use for duct sizing?
0.08 inches of water column per 100 feet is the standard residential friction rate. It balances duct size, blower energy, and noise. Tighter systems with ECM blowers can use 0.10; systems with long runs or many fittings should use 0.06 to avoid undersizing.
Can return duct be smaller than supply?
Return duct should match or exceed supply duct capacity. Many installers size returns one size smaller, which raises static pressure and reduces airflow across the coil. A system moving 1,000 CFM supply needs 1,000 CFM return — not 800 CFM through an undersized return trunk.

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