Selecting the correct duct standard size is the foundation of every HVAC installation. An undersized duct creates excessive velocity, noise, and pressure drop. An oversized duct wastes material, space, and money. This chart covers all standard duct dimensions -- round, rectangular, and flexible -- so you can match the right duct size to your airflow requirement in seconds.
As a flexible duct manufacturer producing every standard size from 3 inches to 24 inches at our Brookshire, TX facility, we have compiled this reference from our production specifications, ASHRAE data, and SMACNA standards.
Standard Round Duct Sizes
Standard round duct sizes follow a consistent series defined by SMACNA and adopted across the HVAC industry. These are the nominal inner diameters -- the dimension that determines airflow capacity:
| Nominal Diameter | Metric (mm) | Cross-Section Area (sq in) | CFM at 600 FPM | Common Application |
|---|---|---|---|---|
| 3" | 75 | 7.07 | 29 | Bathroom exhaust, range hood |
| 4" | 100 | 12.57 | 52 | Bathroom fan, dryer vent |
| 5" | 125 | 19.63 | 82 | Small room supply, portable AC |
| 6" | 150 | 28.27 | 118 | Standard room supply/return |
| 7" | 175 | 38.48 | 160 | Medium room supply |
| 8" | 200 | 50.27 | 209 | Large room supply, range hood |
| 9" | 225 | 63.62 | 265 | Multi-room branch |
| 10" | 250 | 78.54 | 327 | Main trunk duct, large room |
| 12" | 300 | 113.10 | 471 | Main trunk duct, return air |
| 14" | 350 | 153.94 | 641 | Main trunk, light commercial |
| 16" | 400 | 201.06 | 838 | Commercial supply/return |
| 18" | 450 | 254.47 | 1,060 | Commercial main duct |
| 20" | 500 | 314.16 | 1,309 | Large commercial trunk |
| 24" | 600 | 452.39 | 1,885 | Large commercial/industrial |
Note: CFM values shown are at 600 feet per minute (FPM) velocity, which is the ASHRAE-recommended maximum for residential supply duct. Commercial systems may operate at 800-1,200 FPM, increasing capacity proportionally but also increasing noise and pressure drop. See our flex duct CFM chart for velocity-based capacity at different speeds.
Flex Duct Outside Diameters with Insulation
When planning installations with insulated flex duct, you must account for the insulation thickness that adds to the outside diameter. This is critical for clearance in joist bays, chases, and soffits:
| Inner Diameter | Non-Insulated OD | R4.2 OD | R6 OD | R8 OD |
|---|---|---|---|---|
| 4" | 4.5" | 6.0" | 7.2" | 10.2" |
| 5" | 5.5" | 7.0" | 8.2" | 11.2" |
| 6" | 6.5" | 8.0" | 9.2" | 12.2" |
| 7" | 7.5" | 9.0" | 10.2" | 13.2" |
| 8" | 8.5" | 10.0" | 11.2" | 14.2" |
| 10" | 10.5" | 12.0" | 13.2" | 16.2" |
| 12" | 12.5" | 14.0" | 15.2" | 18.2" |
| 14" | 14.5" | 16.0" | 17.2" | 20.2" |
For a detailed comparison of insulation thicknesses and R-value performance, see our R6 vs R8 duct insulation guide.
Standard Rectangular Duct Dimensions
Rectangular duct is specified by width x height. Standard sizes follow a 2-inch increment series. The most common residential and light commercial rectangular duct sizes are:
| Width x Height | Cross-Section Area (sq in) | Equivalent Round Diameter | CFM at 600 FPM |
|---|---|---|---|
| 8" x 6" | 48 | 7.4" | 200 |
| 10" x 6" | 60 | 8.4" | 250 |
| 10" x 8" | 80 | 9.8" | 333 |
| 12" x 8" | 96 | 10.7" | 400 |
| 14" x 8" | 112 | 11.6" | 467 |
| 16" x 10" | 160 | 13.9" | 667 |
| 20" x 10" | 200 | 15.5" | 833 |
| 24" x 12" | 288 | 18.6" | 1,200 |
Aspect Ratio Rule
SMACNA recommends keeping the aspect ratio (width to height) at 4:1 or less for rectangular duct. Higher aspect ratios increase friction loss and require heavier gauge metal with more reinforcement. A 24" x 6" duct (4:1 ratio) is acceptable; a 36" x 6" duct (6:1) is not recommended. For more detail, see our rectangular duct guide.
Round to Rectangular Equivalent Chart
When transitioning from round flex duct to rectangular register boots or trunk lines, you need the equivalent rectangular size. Two ducts are "equivalent" when they produce the same friction loss at the same airflow. The equivalent diameter formula is:
De = 1.30 x ((a x b)^0.625) / ((a + b)^0.25)
Where De is the equivalent round diameter, a is the width, and b is the height of the rectangular duct. Here are common conversions:
| Round Diameter | Equivalent Rectangular | Alternate Size |
|---|---|---|
| 6" | 8" x 4" | 6" x 6" |
| 8" | 10" x 6" | 8" x 8" |
| 10" | 12" x 8" | 14" x 6" |
| 12" | 16" x 8" | 14" x 10" |
| 14" | 18" x 10" | 16" x 12" |
| 16" | 22" x 10" | 18" x 14" |
CFM Capacity by Duct Size
The following table shows the recommended CFM range for each standard duct size, based on ASHRAE-recommended velocities for residential (600 FPM max) and commercial (1,000 FPM max) systems:
| Duct Size | Residential Max CFM (600 FPM) | Commercial Max CFM (1,000 FPM) | Flex Duct Max CFM (500 FPM) |
|---|---|---|---|
| 4" | 52 | 87 | 44 |
| 6" | 118 | 196 | 98 |
| 8" | 209 | 349 | 175 |
| 10" | 327 | 545 | 273 |
| 12" | 471 | 785 | 393 |
| 14" | 641 | 1,069 | 534 |
| 16" | 838 | 1,396 | 698 |
| 18" | 1,060 | 1,767 | 884 |
| 20" | 1,309 | 2,182 | 1,091 |
| 24" | 1,885 | 3,142 | 1,571 |
Flex duct has lower maximum velocities than rigid duct because corrugated walls create more friction and noise. For detailed flex duct sizing, use our online ductulator calculator.
Duct Size by Room Size
For quick residential sizing without a Manual D calculation, use this rule-of-thumb chart based on 1 CFM per square foot of floor area (typical for well-insulated homes in moderate climates):
| Room Size (sq ft) | Approx. CFM Needed | Recommended Duct Size |
|---|---|---|
| Up to 50 sq ft | 50 | 4" |
| 50 - 100 sq ft | 50 - 100 | 5" or 6" |
| 100 - 200 sq ft | 100 - 200 | 6" or 7" |
| 200 - 300 sq ft | 200 - 300 | 8" or 9" |
| 300 - 450 sq ft | 300 - 450 | 10" or 12" |
These are starting points only. Actual duct sizing depends on climate zone, insulation level, window area, occupancy, and system type. For accurate sizing, follow ACCA Manual D or use our HVAC duct sizing guide.
Clearance Requirements
When routing duct through framing cavities, always verify clearance with the outside diameter (not the nominal inner diameter):
- 2x8 joist bay: 7.25" clear height. Fits up to 6" non-insulated flex or 4" R6 insulated duct.
- 2x10 joist bay: 9.25" clear height. Fits up to 8" non-insulated flex or 6" R6 insulated duct.
- 2x12 joist bay: 11.25" clear height. Fits up to 10" non-insulated flex or 8" R6 insulated duct.
- Open truss attic: Variable height. Measure at the narrowest point along the duct route. Remember that insulated duct should not be compressed -- compressed insulation loses R-value.
When to Upsize Your Duct
Always go up one size (never down) when you are between sizes. Additionally, upsize in these situations:
- Long runs (over 25 feet). Friction loss accumulates over distance. A run that needs 6" for a 10-foot branch may need 7" or 8" at 30 feet to maintain adequate airflow.
- Multiple bends. Each 90-degree bend in flex duct adds approximately 15-20 equivalent feet of friction. A 15-foot run with three bends has the friction of a 60-75 foot straight run.
- Flex duct instead of rigid. Flex duct generates 30-50% more friction than rigid duct at the same diameter. When replacing a rigid branch with flex, consider going up one size.
- Noise-sensitive rooms. Lower velocity means quieter operation. Upsizing one diameter reduces velocity by 30-40% and noise by approximately 5-8 dB.
Ordering Standard Sizes from USFlexDuct
We stock all standard duct sizes from 3 inches to 24 inches in non-insulated, R4.2, R6, and R8 configurations. Standard length is 25 feet per piece. Custom lengths and diameters up to 48 inches are available for OEM and large-quantity orders.
Browse our complete product line:
- Non-Insulated Flexible Ducts -- aluminum, polyester, PVC, and semi-rigid
- Insulated Flexible Ducts -- R4.2, R6, R8, and soundproof
- Industrial Hoses -- nylon, canvas, silicone, PU, PVC
- Duct Accessories -- connectors, tape, hangers, flexible connectors
Need a size not listed? Contact us for custom sizing. We quote within 24 hours and ship from our Brookshire, TX warehouse.