Duct Size Calculator and CFM Chart for Ductwork
At the common design friction rate of 0.1 in. of water per 100 ft, 400 CFM needs a 9.8″ round duct, so a 10″ duct running at about 730 ft/min. The rectangular equivalent is 11 × 8″. A 3-ton system (1,200 CFM) needs about a 16″ round trunk at 0.08.
How duct size is calculated
Duct sizing is about balance. The blower pushes a fixed amount of pressure, and every foot of duct and every fitting uses some up. The friction rate is how much pressure you allow per 100 ft of straight duct. The calculator finds the smallest round duct that carries your airflow at that rate. It uses the fitted version of the ASHRAE friction chart for galvanized steel and standard air:
D = (0.109136 × CFM1.9 ÷ friction rate)1/5.02
The exact diameter is then rounded up to the next standard size (4, 5, 6, 7, 8, 9, 10, 12, 14″ …). The real friction shown in the readout is the rate at that standard size, which is always at or below your target. Velocity is CFM ÷ duct area in square feet.
Rectangular ducts are matched by equivalent diameter, the Huebscher equation, so a rectangle and a round duct with the same friction at the same airflow are interchangeable:
A rectangle always needs more sheet metal than its round equivalent, and very flat ducts (wider than 4:1) waste the most. Keep the aspect ratio under about 4:1 where you can.
Which friction rate to use
The 0.1 rule is a shortcut that works for short, simple systems. ACCA Manual D works it out properly: take the blower's available static pressure, subtract the coil, filter and grilles, and divide what is left by the total effective length, which is the longest supply run plus the longest return run, including the equivalent length of every elbow and boot.
Example: 0.50 in. available, minus 0.20 for the coil, 0.10 for the filter and 0.06 for the supply and return grilles, leaves 0.14 in. With a total effective length of 175 ft, the friction rate is 0.14 × 100 ÷ 175 = 0.08. Long runs, cheap filters and many elbows push the rate down to 0.05–0.06, which means bigger ducts.
CFM chart for round duct at three friction rates
Most duct charts give one CFM value per size without saying which friction rate it assumes. The answer for a 6″ duct changes by about 40% between 0.05 and 0.10. This chart shows all three rates, plus velocity at 0.08.
| Round duct | CFM at 0.05 | CFM at 0.08 | CFM at 0.10 | fpm at 0.08 |
|---|---|---|---|---|
| 4″ | 25 | 35 | 35 | 380 |
| 5″ | 45 | 60 | 65 | 440 |
| 6″ | 75 | 95 | 110 | 490 |
| 7″ | 115 | 145 | 165 | 540 |
| 8″ | 160 | 205 | 230 | 590 |
| 9″ | 220 | 280 | 315 | 640 |
| 10″ | 290 | 370 | 420 | 680 |
| 12″ | 470 | 605 | 680 | 770 |
| 14″ | 710 | 905 | 1,020 | 850 |
| 16″ | 1,005 | 1,290 | 1,450 | 920 |
| 18″ | 1,375 | 1,760 | 1,980 | 1,000 |
| 20″ | 1,815 | 2,325 | 2,615 | 1,070 |
Rectangular duct sizes and CFM
| Rectangular | Equivalent round | CFM at 0.08 | CFM at 0.10 |
|---|---|---|---|
| 8 × 6″ | 7.6″ | 180 | 200 |
| 10 × 6″ | 8.4″ | 235 | 265 |
| 10 × 8″ | 9.8″ | 350 | 395 |
| 12 × 8″ | 10.7″ | 440 | 495 |
| 14 × 8″ | 11.5″ | 535 | 600 |
| 16 × 8″ | 12.2″ | 630 | 705 |
| 12 × 10″ | 12.0″ | 600 | 675 |
| 20 × 8″ | 13.5″ | 820 | 920 |
| 20 × 10″ | 15.2″ | 1,135 | 1,275 |
| 24 × 10″ | 16.5″ | 1,410 | 1,585 |
| 24 × 12″ | 18.3″ | 1,835 | 2,060 |
| 30 × 12″ | 20.2″ | 2,395 | 2,690 |
Trunk size by AC tonnage
With the airflow from the AC size calculator (400 CFM per ton), these are the main supply trunk sizes at a 0.08 friction rate. The return trunk carries the same air and is usually sized one step larger, or at a lower friction rate, to keep it quiet.
| System | CFM | Round trunk | Rect, 8″ tall | fpm |
|---|---|---|---|---|
| 1.5 t | 600 | 12″ | 16 × 8″ | 760 |
| 2 t | 800 | 14″ | 22 × 8″ | 750 |
| 2.5 t | 1,000 | 16″ | 30 × 8″ | 720 |
| 3 t | 1,200 | 16″ | 30 × 8″ | 860 |
| 3.5 t | 1,400 | 18″ | 40 × 8″ | 790 |
| 4 t | 1,600 | 18″ | 40 × 8″ | 910 |
| 5 t | 2,000 | 20″ | 51 × 8″ | 920 |
Velocity and noise limits
A duct can meet the friction rate and still whistle. After sizing, check velocity against these common residential targets:
- Supply trunk: 700–900 ft/min.
- Supply branch: 500–700 ft/min, lower near bedrooms.
- Return trunk and grilles: 500–700 ft/min, because return noise travels straight into the room.
If the velocity is over the target, go up one size even though the friction rate says you could stay smaller.
Flex duct: why it carries less air
Flexible duct has a corrugated inner liner, so it has more friction than smooth metal. Stretched tight and straight, it comes close to metal. Left even slightly compressed, its friction rises sharply, and sags and sharp bends make it worse. Our rule: when you tick "flex duct", the calculator shows the next standard size up, and you should keep runs short, straight and supported every 4–5 ft. As a field rule of thumb, a 6″ flex branch is good for about 75–100 CFM in a typical install, not the 110 CFM a 6″ metal duct carries at 0.10.
How many CFM does each room need?
Split the system airflow by each room's share of the cooling load, not its floor area. A sunny room with a big west window takes more than its square footage suggests. As a first pass, take the room's BTU from the BTU calculator, divide by the whole-house load, and multiply by the system CFM. Then pick the branch size from the round-duct chart.
Ducts in the attic: leakage and insulation
A perfectly sized duct still wastes energy if it leaks or sits bare in a hot attic. In summer, attic air can pass 130 °F, so a supply duct there gains heat along its whole length, and a 3-ton system can lose a noticeable part of its capacity before the air reaches a room. The 2021 IECC (section R403.3) requires supply and return ducts in attics to be insulated to at least R-8 when they are 3 inches or larger in diameter, and R-6 when smaller. Seal every joint with mastic or listed foil tape before you wrap it; duct tape dries out and lets go. If you are adding attic insulation, the R-value chart shows how deep to go so the ducts end up buried in it rather than lying on top.
Return ducts deserve the same care. A leaky return in the attic or garage pulls in hot, dusty air, which shows up as a coil that never gets cold enough and as dust around the supply grilles.
Sources
- ASHRAE Handbook of Fundamentals, Duct Design chapter: friction chart and equivalent diameter (Huebscher).
- Engineering ToolBox: fitted friction equation Δh = 0.109136 q1.9 / de5.02 and equivalent diameter tables (engineeringtoolbox.com), checked November 14, 2026.
- ACCA Manual D, Residential Duct Systems: friction-rate worksheet and effective length method.
Questions people ask
How much CFM does a 12 inch duct move?
About 605 CFM at a 0.08 friction rate and about 680 CFM at 0.10, for smooth metal duct. Flex duct of the same size carries less.
How many CFM does a 6 inch duct carry?
About 95 CFM at 0.08 and about 110 CFM at 0.10 for metal. A 6-inch flex branch in a typical install is safer at 75–100 CFM.
How many CFM does a 5 inch flex duct have?
A 5″ metal duct carries about 60 CFM at 0.08. Flex of the same size is usually good for about 50–65 CFM, depending on how tightly it is stretched.
How many CFM does a 10x10 duct carry?
A 10 × 10″ duct is equivalent to a 10.9″ round duct, so about 470 CFM at 0.08 friction.
What are the standard sizes for round ducts?
Residential round duct is commonly stocked in 4, 5, 6, 7, 8, 9, 10, 12, 14 and 16 inch diameters, with larger trunk sizes in 2-inch steps. An 8″ duct carries about 205 CFM at 0.08.
How many CFM per ton of AC?
About 400 CFM per ton, or about 350 in humid climates where the system is set up to remove more moisture.
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