Round to rectangular duct, at equal friction.

Short answer

The equal-friction equivalent diameter of a rectangular duct is 1.30 x (a x b) to the 0.625, divided by (a + b) to the 0.25, with the sides a and b in inches. A 10 x 16 in rectangular duct is equivalent to a 13.7 in round duct. A 10 in round duct with one side fixed at 8 in needs about 10.5 in on the other side.

Worked example

A 10 in round duct has to fit under a joist bay that limits one side to 8 in. Solving the equation for the other side gives 10.51 in, so an 8 x 11 in rectangular duct gives the same friction at the same airflow, with slightly less friction because 11 is a little over the exact answer. Going the other way, a 10 x 16 in duct converts to 13.7 in round.

Formula, assumptions and sources

ASHRAE credits Huebscher (1948) with the relationship between rectangular and round duct that gives equal length, friction and airflow, and uses it as the basis for its equivalent-diameter table in the Duct Design chapter. The tool applies that relationship, De = 1.30 (ab)^0.625 / (a + b)^0.25, and for the round-to-rectangular direction it solves the same equation for the second side by iteration. Equal friction at equal airflow does not mean equal area: a flat rectangle usually has more cross-section than the round duct, so its air speed is lower. Use the Duct Velocity and Friction Loss Calculator to check the speed of the size you pick. ASHRAE also notes that its pressure loss must still be calculated separately; this tool only converts the size.

Source: ASHRAE Handbook - Fundamentals (2017), Chapter 21: Duct Design (retrieved 2026-10-15)

Safety and limits

This is a planning aid, not professional advice. Duct, furnace and air-conditioner design affects comfort, combustion safety and code compliance; a licensed HVAC contractor should perform a load calculation, size the system and verify airflow on site.

FAQ

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

Choose one side of the rectangle, usually the one limited by the space available, then solve the equal-friction equation for the other side. This tool does that for you.

Is a rectangular duct with the same equivalent diameter the same area?

No. The equivalent diameter matches friction and airflow, not area. A flat rectangle has a larger cross-section than the round duct it replaces.

Which equation does this use?

The Huebscher equation, De = 1.30 (ab)^0.625 / (a + b)^0.25, which ASHRAE uses for its table of circular equivalents of rectangular duct.

Is an 8 x 8 inch duct the same as an 8 inch round duct?

No. An 8 x 8 in square duct is equivalent to about 8.7 in round for friction, because the square has more cross-section than an 8 in circle.