Equivalent diameter vs. hydraulic diameter: which one does your duct problem want?

Run both formulas on the same rectangular duct and you get two different numbers — which is the whole point, and the whole trap. They're not competing approximations of one idea; they answer two different questions, and duct problems are written to see if you know which question you're in.

Hydraulic diameter (Dh = 4A/P) is the general fluid-mechanics tool: four times flow area over wetted perimeter. It's what you use to compute a Reynolds number or apply friction relations directly to a non-circular passage — any shape, any fluid, fully general. It answers: "what length scale does the flow physics see?"

Equivalent (round) diameter (De) is the HVAC duct-sizing tool: the diameter of a round duct that would have the same friction loss and airflow as your rectangular one. It exists so you can take a rectangular duct into round-duct friction charts and tables — it answers: "which row of the chart is my duct?" It comes from an empirical relation, and it will not equal Dh for the same duct. One search tip for the on-screen Handbook: its label for De is "circular equivalent" — search that phrase, not "equivalent diameter."

The tell is the workflow, not the chapter: if you're computing Re or plugging into a friction-factor equation yourself → Dh. If you're entering a friction chart, duct-sizing table, or "equivalent round size" workflow → De. HVAC&R problems lean De because duct design leans on charts; TFS-style fluids problems lean Dh because they run the equations. Don't expect the Handbook's layout to disambiguate for you — the two definitions sit in adjacent subsections of the same duct-design pages, so the question you're answering, not the section you're in, is what picks the diameter.

Why the exam loves it: the two values are close enough to be plausible and different enough to change the answer — a purpose-built distractor pair. It's a one-sentence distinction that's worth one full problem's points. If you hit a duct problem where the choice still feels murky, this is precisely the kind of "which relation and why" question Castor answers on the spot, citing the Handbook section the problem should run through. Start your free trial — 7 days, no credit card.