Rectangular to Round Duct Conversion Chart (Equivalent Diameter Explained)

Duct sizing · Equivalent diameter · Updated 2026-07-22

A 14x6 rectangular duct is the equivalent of a 9.8 in. round duct, a 12x8 is a 10.7 in. round, and a 30x10 is an 18.3 in. round. Those numbers come from the Huebscher (1948) circular-equivalent relationship published as ASHRAE Handbook - Fundamentals, Chapter 21, Eq. 1B: De = 1.30 (ab)^0.625 / (a + b)^0.250, where a and b are the rectangular sides in inches.

The full matrix is below, 4 in. to 36 in. wide by 4 in. to 20 in. deep. Read the paragraph above it first, though, because the equation equates the two shapes at equal friction and equal capacity, not equal cross-sectional area - and the field habit of matching area is how a correctly designed branch quietly loses airflow after somebody swaps in a transition.

What equivalent diameter actually means

Circular equivalent diameter is the diameter of a round duct that produces the same pressure drop per foot at the same airflow as the rectangular duct. It is not the round duct with the same open area, and it is not the hydraulic diameter 4A/P. Air in a rectangle pays a penalty: the corners hold slow, sheared air that adds almost nothing to flow and plenty to friction, so a rectangle always behaves like a round duct smaller than its area suggests.

Huebscher's 1948 correlation captures that penalty empirically, and ASHRAE has carried it forward as the basis for the Circular Equivalents of Rectangular Ducts for Equal Friction and Capacity table. Two working limits: the correlation is intended for aspect ratios up to roughly 8:1 (the stated ceiling varies by edition), and it equates friction, not velocity. The rectangle and its round equivalent carry the same CFM at the same friction rate, but their areas differ, so their velocities do too - which matters when you are checking a noise limit.

Rectangular to round duct conversion chart

The equal-area trap, worked

Say a 14 in. round branch has to run through a 6 in. joist bay, so it gets converted to rectangular. A 14 in. round is 153.9 sq in. of free area. Matching area at 6 in. deep gives 26x6 (156 sq in.), which looks like a clean swap and gets fabricated all the time. Run 26x6 through Eq. 1B and it comes out at 12.8 in. equivalent - the branch now has the friction of a 12.8 in. round, not a 14 in. one.

To genuinely match a 14 in. round at 6 in. deep you need 32x6, which is 192 sq in. - about 23% more free area than the equal-area guess. The wider the aspect ratio, the wider the gap, and those percentages stack across a run until the far bedroom is short on air and nobody can find the reason.

How to use the chart in the field

1. Start from the round size the design calls for: Size the run from CFM and a friction rate first. See CFM to duct size and friction rate if that number is not settled.
2. Fix the dimension the building fixes for you: Joist depth, soffit height, or plenum clearance sets one side. That is your b.
3. Read down that depth column: Find the first row meeting or exceeding your target round diameter, then read the width. Round up, never down.
4. Check the aspect ratio before you order: Stay at or below 4:1 where you can. Past that the duct costs more metal and drifts toward the edge of the correlation's range.
5. Re-check velocity, not just friction: A flat rectangle at the same De has more area and lower velocity, which usually helps noise - but verify against the duct velocity chart before signing off a bedroom branch.

Run the ASHRAE equation instead of interpolating a chart. Ductulator computes equivalent round diameter from the Huebscher equation (ASHRAE Fundamentals Ch. 21 Eq. 1B) on device, in 64-point type, alongside eight valid duct sizes annotated with velocity and friction and a SMACNA Table 1-4 gauge pick. Every result cites its source standard, and the exported PDF schedule carries the SMACNA and ASHRAE references on every page. Learn more about Ductulator.

Where the conversion sits in a real duct calculation

Equivalent diameter is one step, not the whole job. The friction rate has to come from available static pressure divided by a total effective length that includes fitting equivalent lengths, and the finished rectangle needs a gauge that will hold its shape. Ductulator chains those together: enter CFM and a friction-rate limit, pick rectangular, round, or flex, and it returns the equivalent round diameter, eight valid sizes with over-velocity and over-friction options flagged in safety red, and the SMACNA Table 1-4 gauge - then drops the run into a multi-segment static pressure budget so cumulative pressure drop renders against the pressure you actually have. Export the job stamped, and when an inspector asks where 9.8 in. came from, the citation is already on the page.

Frequently asked questions

What size round duct is a 14x6 rectangular duct?

9.8 in. Round up to a 10 in. round in practice. That figure is the Huebscher circular equivalent from ASHRAE Fundamentals Ch. 21 Eq. 1B, meaning a 14x6 and a 9.8 in. round carry the same airflow at the same friction loss per foot.

How do you convert rectangular duct to round duct?

Use De = 1.30 (ab)^0.625 / (a + b)^0.250 with a and b as the rectangular sides in inches, or read the chart above. Do not convert by matching cross-sectional area - that overstates the equivalent by roughly 3 to 7% across common residential sizes, and more as the aspect ratio grows.

Is equivalent diameter the same as hydraulic diameter?

No. Hydraulic diameter is 4A/P, a geometric definition used broadly in fluid mechanics. Circular equivalent diameter is the empirical Huebscher result that equates a rectangle to a round duct at equal friction and equal capacity. For a 12x8 the hydraulic diameter is 9.6 in. while the circular equivalent is 10.7 in., so the two are not interchangeable.

Does a rectangular duct or a round duct move more air?

For the same equivalent diameter they move the same air at the same friction, but the round duct does it with less material: an 18.3 in. round needs about 28% less perimeter per foot than the 30x10 rectangle it replaces. Rectangular duct earns its place when depth clearance is limited, not when performance is the question.

Keep reading

• CFM to Duct Size Chart: What Size Duct Do I Need for 400 CFM?
• Friction Rate in HVAC: What It Is and How to Calculate It
• Duct Velocity Chart: Recommended FPM for Supply, Return, and Branch Ducts