Round to Rectangular Duct Converter
Turn a round duct into a rectangular one with the same friction, or a rectangle into its round equivalent.
How to use ↓Result
Duct Chart · Round to Rectangular Duct Converter · ductchart.com
Estimates only: they can differ from your chart or the manufacturer's data by 5% or more (flex by 2 times or more), so verify accuracy for your own application against the design documents, manufacturer data and local code.
How to use
- Choose Round to rectangular or Rectangular to round.
- Round to rectangular: enter the diameter and the side you must keep, usually the height.
- Add the airflow if you want the velocity in the rectangle.
- Press Calculate. Check the aspect ratio before you build it.
Round to rectangular duct sizes
A rectangular duct and a round duct have the same friction at the same airflow when their equivalent diameters match. This converter uses the standard equal-friction equation: equivalent diameter = 1.30 × (a × b)0.625 ÷ (a + b)0.25, where a and b are the sides in inches. It matches the ASHRAE circular equivalents table to within about 1 percent plus the rounding in the printed table.
Going from round to rectangular, the calculator solves for the other side, then rounds it up to the next whole inch so the rectangle is never smaller than the round. The table underneath shows the same round duct at heights from 4 to 14 in.
Equal friction does not mean equal velocity. The rectangle has more cross-section area than the round it matches, so the air moves slower in it. Add the airflow and the converter shows both velocities.
Keep the aspect ratio at 4:1 or less when you can. Tall, thin ducts mean more sheet metal, more leakage and more noise, so the calculator flags anything over 4:1. For a full job, size the round duct first with the duct size calculator.
Example: an 8 × 12 in. duct has the same friction as a 10.7 in. round. A 10.2 in. round with an 8 in. height becomes 8 × 11 in. At 400 CFM the 8 × 11 in. rectangle runs at 655 fpm, against 705 fpm in the 10.2 in. round.
FAQ
Does the rectangle carry the same airflow as the round?
It has the same friction at the same airflow, so it can replace the round duct without changing the pressure drop. The velocity is lower because the area is larger.
Why is the other side rounded up?
Rounding up keeps the rectangle at or above the round duct’s equivalent size. The exact side is shown as well if you want to see it.
What is the best aspect ratio?
As close to square as the space allows. Over 4:1 gets a warning here because of the extra sheet metal, leakage and noise.