How do you convert a rectangular duct to a round duct?

Use the ASHRAE equal friction equivalent diameter formula: De = 1.30 x (a x b)^0.625 / (a + b)^0.25, where a and b are the rectangular duct dimensions in inches and De is the equivalent round diameter in inches. This gives you a round duct with the same pressure drop per foot as the rectangular duct at the same airflow. For example, a 12x8 inch rectangular duct converts to De = 1.30 x (12x8)^0.625 / (12+8)^0.25 = approximately 10.2 inches round. Round up to the nearest standard size (10.5 or 11 inches).

Is a round duct better than rectangular for the same airflow?

Yes. Round ducts are more aerodynamically efficient than rectangular ducts because they have no corners where turbulence forms. For the same cross-sectional area, a round duct has lower friction loss per foot, lower surface area (less heat gain or loss), less material cost, and easier installation with flexible connections. The only advantage of rectangular duct is fitting into constrained spaces where depth is limited — like between floor joists or in shallow ceiling cavities. Whenever space permits, convert to round or flat oval duct for better performance.

⬛ Rect to Round Converter

Rectangular to Round Duct Calculator

Find the round duct diameter that matches a rectangular duct for equal friction loss. Also converts round to rectangular with any height constraint. Used when replacing sheet metal with flex duct, or designing hybrid duct systems.

Unit System:

⬛ Rectangular Duct Dimensions

Width (a) *

Height (b) *

Airflow (for velocity check)

CFM

⬤ Round to Rectangular (reverse)

Round Diameter i

Max Rectangular Height i

⬛ Duct Conversion Results

Property	Rectangular	Round Equivalent

Export:

When should you switch from rectangular to round duct?

If you are replacing an old sheet metal duct system or adding branches to an existing system, converting to round flex duct is almost always the right call when space permits. Round duct has lower friction loss, lower material cost, easier installation, fewer leakage points, and is available in standard diameters that work with almost any fitting. The only time rectangular is preferred is when ceiling or floor joist depth limits one dimension and you need to maintain a specific height while maximizing airflow.

ASHRAE equal friction formula explained

The ASHRAE formula De = 1.30 x (a x b)^0.625 / (a + b)^0.25 gives the equivalent round diameter for equal friction loss per unit length. This is not the same as equal cross-sectional area. A 12x8 rectangular duct has 96 in² of area, which would be equivalent to a 11.1-inch round duct by area alone. But the ASHRAE equivalent diameter is 10.2 inches — smaller — because the rectangular duct's corners and higher surface-area-to-volume ratio create more friction than a round duct of the same area. The friction-equivalent round duct is always smaller than the area-equivalent round duct.

Aspect ratio and duct efficiency

Rectangular duct efficiency degrades rapidly as aspect ratio (width to height) increases. A 4:1 ratio duct has about 25% more friction per unit length than a round duct of the same area. A 8:1 ratio duct has 60% more friction. SMACNA and ASHRAE recommend keeping aspect ratios at 4:1 or less. Wide, flat ducts that fit between joists (like 20x4) are extremely inefficient — consider flat oval duct as a better alternative in these situations.

Frequently Asked Questions

Can I use flex duct to replace a rectangular sheet metal run?+

Yes, but use this calculator to find the correct flex duct diameter first. Then add one duct size to account for flex duct's higher roughness compared to sheet metal. If the rectangular duct converts to a 9-inch equivalent round, use 10-inch flex duct. Also ensure the flex duct is fully extended — a compressed or kinked flex duct has 3-5 times the pressure drop of a fully extended one. Standard duct design practice limits flex duct runs to 6 feet maximum for branch ducts. For longer runs, use sheet metal with flex only at the final connection to the register boot.

What is the standard round duct size in Canada?+

Standard round duct diameters available in Canada: 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 20 inches. Flex duct is most commonly stocked in 4, 6, 8, 10, and 12 inches at HVAC suppliers. When this calculator gives you an equivalent diameter that falls between standard sizes, always round up to the next standard size — never down. Rounding down results in higher velocity and friction than designed, reducing airflow to that room. The cost difference between one duct size is minimal compared to the callbacks from a room that doesn't heat or cool properly.