Custom Square-to-Round Duct Fittings, Made to Order
Yes. A square-to-round (or rectangular-to-round) fitting connects a rectangular duct or plenum to round pipe, and the two openings almost never match a stock size. Enter the rectangular inlet width and depth, the round outlet diameter, and the length, and the flat pattern is developed and CNC-cut so the finished piece meets both openings exactly — no field snips, no caulk gap.
The hardest fitting to make by hand
A square-to-round is the fitting field crews least want to fabricate. Transitioning a flat-sided rectangle into a true circle over a fixed length is a genuine developed-surface layout — get the pattern slightly wrong and it either won't close or it collars the round pipe unevenly.
Ordering it made-to-order moves that layout onto a CNC table. The transition surface is developed mathematically from your three inputs, so the round end is actually round and the rectangular end actually matches your duct.
What to specify
- Rectangular inlet — width × depth of the duct or plenum side
- Round outlet — the pipe or collar diameter it feeds
- Length — the transition distance; longer is gentler on airflow
- Centered or offset — round outlet centered, or shifted to line up with existing pipe
- Gauge and material — 26–20 gauge galvanized, aluminum or stainless
- Connection type — slip/drive on the rectangular end, raw or beaded collar on the round end
Where they show up
Square-to-rounds are everywhere you leave a rectangular trunk or plenum for round branch pipe: furnace and air-handler supply/return collars, rooftop unit tie-ins, and any spot a rectangular main feeds spiral or snap-lock round. Because each of those openings is dictated by equipment you didn't pick, the fitting between them is almost always a custom size.
Configure Your Square-to-Round
Enter the rectangular and round openings, see the price instantly, and ship it to the job.
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