Equivalent Duct Diameter Calculator

Convert a rectangular duct to the round duct with the same friction loss, or find the rectangular size that matches a round duct when one side is fixed by a joist space or ceiling cavity.

Width (inches)

Height (inches)

Equivalent Round Diameter

—

Enter values above and click calculate.

How It Works

A rectangular duct does not move air like a round duct of the same area: the corners add friction. The equivalent diameter is the round duct that has the same friction loss at the same airflow.

De = 1.30 × (W × H)^0.625 ÷ (W + H)^0.25

This is the Huebscher equation used in ASHRAE Fundamentals and on ductulators. For example, a 12" × 12" duct is equivalent to a 13.1" round, not the 13.5" round of equal area.

Round to rectangular

Enter the round size and the side you're limited to. The calculator solves for the other side and rounds it up to the next even inch (or 50 mm), so the rectangular duct is never smaller than the round one.

Aspect ratio

Keep rectangular ducts under 4:1. Flatter ducts need more metal and lose more pressure for the same airflow.