DESIGN // BOISE, ID 208-421-4493
Sign In
FREE TOOL // DUCT SIZING

Round ↔ Rectangle Duct Size Converter

Accurate duct equivalency conversions using the ASHRAE equivalent diameter formula. Built for contractors in the field who need fast, reliable answers.

Enter a round duct diameter and get all equivalent rectangular sizes, sorted by aspect ratio. Closest to 1:1 is most efficient.

inches
limit results
Equivalent Rectangular Sizes
Width × Height (in) Equiv. Round (in) Difference Aspect Ratio Area (in²)
Tip: Aspect ratios closer to 1:1 are more aerodynamically efficient and produce less noise. Avoid ratios greater than 4:1 when possible. All conversions use equivalent friction loss per unit length (ASHRAE method).

Enter rectangular duct dimensions to find the equivalent round duct diameter and the nearest standard round sizes.

inches
×
inches
—
Equivalent Round Diameter Based on ASHRAE De = 1.30 × (a×b)^0.625 / (a+b)^0.25
Nearest Standard Round Sizes
Round Size (in) Difference from Exact Effect on Flow Notes
Round up or down? Round up to the next standard size to maintain or slightly increase airflow capacity. Round down only if you need to reduce flow in that branch. Consult your Manual D calculations before finalizing.

What is Equivalent Diameter?

The equivalent diameter is the round duct size that produces the same friction loss per unit length as the rectangular duct — not the same cross-sectional area.

Why Aspect Ratio Matters

High aspect ratios (e.g. 4:1 or greater) increase turbulence, noise, and friction. A 12×12 round is more efficient than an 18×8 even at the same equivalent diameter.

Need a Full Manual D?

This tool handles quick conversions. For a complete duct design with friction rates, velocity checks, and equipment sizing, get a Manual D from us.

ASHRAE Equivalent Diameter Formula

De = 1.30 × (a × b)^0.625 / (a + b)^0.25
De = equivalent round duct diameter (inches)
a = width of rectangular duct (inches)
b = height of rectangular duct (inches)
Source: ASHRAE Fundamentals Handbook, Duct Design chapter

Need a Full Manual D Duct Design?

Quick conversions are a start. A proper Manual D gives you friction rates, velocity checks, and a system that's balanced from the unit to every register.

Get a Quote