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DuctCalc

Round to rectangular duct equivalent, with real velocities

A rectangular duct is equivalent to a round one when both lose the same pressure per foot at the same airflow. At 400 CFM and 0.1 in.wg/100ft the 9.7 in round duct becomes a 10 x 8, a 14 x 6 or a 24 x 4.

3 min read · Updated 2026-10-09

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When the duct has to fit a joist bay or a soffit, you pick the height first and need the width that goes with it. DuctCalc lists those pairs under the round answer, so the conversion is already done when you get to the ladder.

Where do the rectangular sizes appear?

Size a duct as usual. Under the round result there is a card called Rectangular equivalents. Each row shows a width by height, the real air velocity in that rectangle, and the De of the pair after rounding.

400 CFM, 0.1 in.wg/100ft, galvanized steel
RectangularRatioVelocityDe of the pair
10 x 8 in1.25:1720 fpm9.8 in
14 x 6 in2.33:1686 fpm9.8 in
24 x 4 in6:1600 fpm9.8 in

The 24 x 4 row carries an orange 6:1 chip. The app flags anything past 4:1, and it drops pairs beyond 8:1 altogether.

How is the equivalent worked out?

DuctCalc uses the equivalent-diameter formula printed on the back of many wheels: De = 1.30 x (a x b)^0.625 / (a + b)^0.25. It steps the height through even inches (50 mm in metric), solves the exact width for each, then rounds that width up to the next even inch. Rounding up is deliberate: a rectangle slightly oversized is quiet, one slightly undersized comes up short when the system is balanced.

Why is the rectangular velocity lower than the round one?

At equal friction the rectangle has more cross-section than the round duct it replaces, so the air moves slower in it. The exact 9.7 in round runs at 781 fpm, while the 10 x 8 carries the same 400 CFM at 720. The app shows the real rectangular figure, not the round one, because that is the air your duct will carry.

How do you check a rectangle you already have?

  1. Tap Check a duct, then Rectangular duct.
  2. Enter the Airflow, the Width and the Height, for example 400, 12 and 8.
  3. Read Air velocity (600 fpm), Friction rate (0.063 in.wg/100ft) and Equivalent round duct (10.7 in).

That 12 x 8 is bigger than the 400 CFM job needs at 0.1, which is why the friction comes out at 0.063. Nothing wrong with that if the bay is there.

Quick near-square pairs

The Reference tab has a section called Round → rectangular (near-square) for 6 to 20 in rounds, offline. Examples: 12 in round is 12 x 12, 14 in is 14 x 12, 16 in is 16 x 14, 20 in is 20 x 18. It lists the squarest pair only; the calculator lists every workable one.

Questions and answers

What is the rectangular equivalent of a 10 inch round duct?
The Reference tab gives 10 x 10 as the near-square pair. For a flatter shape, size the airflow you have and read the list: at 400 CFM and 0.1 the options include 10 x 8 and 14 x 6.
Is the maximum aspect ratio 4:1?
The app flags ratios above 4:1 with a chip and hides anything above 8:1. Which limit applies on your job is a question for your spec and your code.
Do round and rectangular carry the same airflow at the same speed?
No. They match in friction loss, not in velocity. The rectangle's cross-section is larger, so its velocity is lower.
Can it convert a duct size without an airflow?
Not in the calculator, which needs the airflow for the list. The Reference tab has the near-square table for common rounds without one.

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