HVAC · Air side
Round to Rectangular Duct Calculator
Convert a duct between round and rectangular either direction. Enter a rectangular size to get its equivalent round diameter, or enter a round size to get the rectangular ducts that carry the same airflow at the same friction. Equivalent diameter from ASHRAE Fundamentals 21.3.1 (Huebscher), computed on each build, not looked up from a hand-typed grid.
Round to Rectangular Duct Equivalents
| Round size | Equivalent rectangular ducts (in) |
|---|---|
| 6 in round | 6×6 |
| 8 in round | 10×6 |
| 10 in round | 16×6 |
| 12 in round | 24×6, 16×8 |
| 14 in round | 22×8, 18×10, 14×12 |
| 16 in round | 30×8, 24×10, 16×14 |
| 18 in round | 30×10, 24×12, 20×14, 18×16 |
| 20 in round | 30×12, 22×16 |
Each round size and the stocked rectangular ducts whose equivalent diameter matches it, so each rectangular option carries the same airflow at the same friction. Shallower ducts run wider to hold the same equivalent diameter. To size a duct from CFM first, use the duct size calculator.
The equivalent-diameter formula
De = 1.30 × (a × b)0.625 / (a + b)0.25, where a and b are the rectangular sides in inches and De is the equivalent round diameter in inches (ASHRAE Fundamentals 21.3.1, the Huebscher equation). De is the round diameter that has the same friction loss at the same airflow as the rectangle, so matching equivalent diameters is how you swap shapes without changing the air. The correlation is published for aspect ratios up to about 8:1. To go the other way, this tool solves the same equation for the width at a depth you choose.
Round to Rectangular Duct FAQs
How do I convert round duct to rectangular?
Match the equivalent diameter: the rectangular duct that carries the same airflow at the same friction as a round one is the one whose equivalent diameter De equals the round size. De = 1.30 times (a times b) to the 0.625 power, divided by (a plus b) to the 0.25 power (ASHRAE Fundamentals 21.3.1). For example a 22 by 8 in rectangular duct has an equivalent diameter of about 14 in, so it replaces a 14 in round. This converter solves it both directions for you.
What is the rectangular equivalent of a 14 inch round duct?
A 14 in round duct equals a 22 by 8 in rectangular duct (exact width 21.8 in at 8 in deep), or a squarer option like 18 by 10 in, 14 by 12 in, all at the same friction and airflow. Shallower ducts have to be wider to keep the same equivalent diameter, so pick the depth your joist bay or ceiling allows and read the width off the converter.
What size rectangular duct equals a 10 inch round?
A 10 in round duct equals about a 15 by 6 in rectangular duct (exact 14.6 in wide at 6 in deep); the nearest stocked size is 16 by 6 in. It carries the same air at the same friction as the 10 in round. A shallow 6 in duct has to run wide to hold the equivalent diameter, so step up the depth and the width drops.
What is the equivalent diameter of a 12 by 6 duct?
A 12 by 6 in rectangular duct has an equivalent round diameter of 9.1 in (ASHRAE Fundamentals 21.3.1). That means it carries the same airflow at the same friction loss as a 9.1 in round duct, and you would round up to a 10 in round to fit a stocked size.
Is round or rectangular duct better?
Round duct moves the same air with less friction and less sheet metal than rectangular, so it is the more efficient shape wherever it fits. Rectangular duct wins on fit: it lies flat in joist bays, above ceilings, and inside soffits where a round pipe will not. Size them to the same equivalent diameter and they carry the same airflow at the same friction; the choice is about the space, not the airflow.
How do you calculate equivalent duct diameter?
The equivalent (or equal-friction) diameter of a rectangular duct is De = 1.30 times (a times b) to the 0.625 power, divided by (a plus b) to the 0.25 power, where a and b are the rectangular sides in inches (ASHRAE Fundamentals 21.3.1, the Huebscher equation). It is the round diameter that has the same friction loss at the same airflow, and it is valid to about an 8 to 1 aspect ratio. This tool computes it, and its inverse, on every build.
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Size a duct from CFM by the equal-friction method, then convert its shape here. The air-side start.
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Turn available static pressure and total effective length into the friction rate the sizer needs.