Duct Equivalent Diameter Calculator (Rectangular to Round, Huebscher)
Convert a rectangular HVAC duct to its equivalent round diameter with the Huebscher (ASHRAE) formula — hydraulic diameter, aspect ratio, air velocity, friction loss rate and the nearest standard round size.
How it works
Huebscher (ASHRAE Fundamentals, Duct Design chapter): De = 1.30 × (a·b)^0.625 / (a + b)^0.25 · hydraulic diameter Dh = 2ab/(a + b) · velocity = airflow ÷ area · friction rate for galvanised round duct ≈ 0.109136 × CFM^1.9 / De^5.02 in. w.g. per 100 ft (1 in. w.g./100 ft = 8.17 Pa/m).
A rectangular duct with the same airflow has more friction than a round duct of the same area, because its corners add wetted surface. Huebscher’s equivalent diameter, used in ASHRAE and SMACNA duct design, is the round duct that has the same pressure loss per unit length at the same airflow, so friction charts and ductulators for round duct can be read for rectangular duct. Note that the equivalent diameter is larger than the hydraulic diameter, and that a rectangular duct carrying the same airflow runs at a lower velocity than its round equivalent. Aspect ratios above about 4 : 1 cost more sheet metal and pressure loss for the same capacity.
Worked example
A 400 × 250 mm duct: De = 1.30 × (100,000)^0.625 / (650)^0.25 = 1.30 × 1,333.5 / 5.049 = 343.3 mm, so it performs like a 343 mm round duct (next standard size 355 mm). Its hydraulic diameter is only 307.7 mm and the aspect ratio is 1.60 : 1. Carrying 1,500 m³/h, air moves at 4.17 m/s in the rectangular duct and the friction loss is about 0.74 Pa/m — close to the 0.8–1 Pa/m usual for equal-friction design.
Sources: ASHRAE Handbook — Fundamentals, Chapter 21 Duct Design (Huebscher equivalent diameter) · Hydraulic diameter and equivalent duct diameter — The Engineering ToolBox
Frequently asked questions
Why is the equivalent diameter not the same as for equal area?
Equal area would give the same velocity, but the rectangular duct has a larger perimeter and therefore more friction. Huebscher’s formula finds the round size with equal friction at equal airflow, which is slightly smaller in area than the rectangle — a 400 × 250 duct (0.1 m²) equals a 343 mm round (0.093 m²).
What is the difference between equivalent and hydraulic diameter?
Hydraulic diameter (4 × area ÷ perimeter) gives the same friction at the same velocity and is used in Reynolds number and Darcy calculations. Equivalent diameter gives the same friction at the same airflow and is what duct sizing charts use.
What friction rate should I design ducts for?
The equal-friction method commonly uses 0.08–0.1 in. w.g. per 100 ft (0.65–0.8 Pa/m) for residential and low-pressure commercial systems, staying within velocity limits for noise — about 4–5 m/s (700–900 fpm) in branches.
Does this work for flat-oval ducts?
No — flat-oval ducts use a different equivalent-diameter formula (Heyt and Diaz) in the ASHRAE Handbook. This calculator covers rectangular ducts only.
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This calculator is for educational purposes. Double-check important results before acting on them.