Rocaris 3-1/4" x 10" x 4'' Galvanized Rectangular to Round Duct Transition, 4" Diameter Microwave Exhaust Vent Kit Round Transition for Connecting HVAC Duct Pipe Fittings

A rectangular-to-round transition is one of those fittings you only think about the moment your project grinds to a halt. I picked up the Rocaris rectangular-to-round adapter to connect a 3-1/4" x 10" rectangular outlet to a 4" round duct run for an over-the-range microwave, and then pressed it into service on a bath fan and a small shop exhaust. Across those installs, it did what I needed: bridge dissimilar duct shapes cleanly, hold up to handling, and keep airflow surprisingly smooth for a budget fitting.

What it is and where it fits

This is a galvanized steel transition that steps a standard 3-1/4" x 10" rectangular connection down to a 4" round. On the rectangular side, you fasten through the flange with sheet-metal screws; on the round side, you slip on 4" flex or rigid round and secure with a clamp and foil tape. There’s no hardware in the box, which is common for duct fittings—plan to supply your own screws, clamp, and UL 181 foil tape (or mastic).

Rocaris gives the interior a smooth path, and that matters more than it sounds. A lot of cheaper transitions have sloppy seams or a crimp that sticks into the airstream; this one is smooth enough that I didn’t see the telltale whistling or turbulence you get with rougher sheet metal.

Build quality and design

The adapter is made from galvanized sheet steel with a clean, uniform finish. It’s light enough to be easy to position single‑handed, but stiff enough not to oil-can or deform under normal screw pressure. The seams on my sample were straight and tight, with no daylight showing when held up to a shop light. After installation and taping, I didn’t pick up any leaks with a smoke pencil.

Edges are sharp. That’s not a knock—it’s typical for HVAC fittings—but it’s worth calling out. Wear cut‑resistant gloves, and consider a quick pass with a deburring tool or file on the rectangular flange if you’re working in a cramped cabinet.

The 4" outlet is round and true, not ovalized. Most 4" clamps and rigid pipe slipped on without fuss. The round end is not crimped; plan to slide flex over it or use a short crimped stub if you’re mating to rigid round.

Real-world sizing and fit

Nominal sizing in ductwork can be confusing. The rectangular opening here is listed as 3-1/4" x 10", which is the industry standard size for a lot of microwave adapters, range hood outlets, and trunk takeoffs. On my sample, the insertion area measured just under the nominal 10" width (as expected for a slip fit into standard duct), with a perimeter flange you can screw through.

Two practical notes from installs:

• Some microwave adapters expect the mating piece to slip inside; others want it to slip over. The Rocaris is a “slip-in” style on the rectangular end. If your appliance expects a “slip-over” collar, you may need a short 3-1/4" x 10" start collar or a spacer to achieve a snug fit.
• If you’re marrying this directly to an appliance plate that’s a hair wider, a couple of tricks help: slightly bow the sidewalls out with a hand seamer for a custom fit, or bridge a small gap with foil tape before running your screws. I’ve done both; both seal well when taped.

As with any thin-gauge fitting, shipping can leave a corner bumped. Mine arrived square, but I’ve had similar parts show up with a minor bend—five minutes with a hand seamer brings them back.

Installation notes

On my microwave install, I did the following:

1. Dry-fit the adapter to confirm orientation and clearances.
2. Mark and predrill pilot holes in the flange; use #8 x 1/2" or #8 x 3/4" sheet-metal screws.
3. Drive four screws, one per side, snug but not crushing the metal.
4. Seal the joint with UL 181 foil tape, burnish it down with a plastic squeegee.
5. Slide 4" flex over the round end, add a worm-drive clamp, and tape over the clamp for an air-tight seal.
6. Strap the round duct so the adapter isn’t carrying the duct weight.

For bathroom fans or small exhaust setups, the steps are the same. For dryer runs, I’d use rigid 4" metal duct, keep screws short so they don’t protrude into the airstream, and avoid plastic or vinyl flex to stay in line with best practices and most local codes.

Performance and airflow

The smooth interior pays dividends. With the microwave running at typical over-the-range CFM levels, the airflow felt even, and noise didn’t spike at the transition. On a test rig with a small inline fan, static pressure increase across the adapter was modest—as you’d expect from a single shape change and smooth wall. There are more efficient transitions out there with longer tapers, but they take more space; this one is compact, and that’s often the constraint inside cabinets and soffits.

The big caveat is system sizing. A 4" round is appropriate for many OTR microwaves, bath fans, and light-duty exhausts. It is not the right choice for higher-CFM range hoods that specify a 6" or 8" round—this adapter will become the bottleneck. Check the appliance manual: if it calls for 6" round, use a 6" transition to avoid noise and performance losses.

Use cases that suit it well

• Over-the-range microwaves vented to 4" round
• Bathroom fans stepping to a 4" exterior hood
• Light workshop exhaust where space above a cabinet or soffit is tight
• Rectangular trunk takeoff to a 4" branch in a retrofit

It’s also fine for short exterior runs. The galvanization will resist corrosion in normal indoor/outdoor residential use. Just remember this is a transition, not an exterior termination; you still need a proper exterior hood with a backdraft damper.

What could be better

• No hardware in the box. I’m used to that with duct fittings, but first-time DIYers might expect screws or a clamp.
• Edges are sharp. A hemmed flange would be friendlier, especially for cabinet work, though it would add cost.
• Rectangular end is a slip-in profile. That’s ideal for standard duct, but some appliances are finicky about needing a slip-over style. A note in the listing or a simple sketch in the box would cut down on guesswork.

If you’re on the fence about fit, measure the actual opening on your appliance and note whether it wants the mating piece inside or outside. A cheap set of calipers and a quick paper template have saved me more returns than I can count.

Value and alternatives

For the price, the Rocaris adapter hits the sweet spot: sturdy enough, square enough, and smooth enough to perform like it should without taxing your budget. You can find heavier-gauge transitions from HVAC wholesalers, and you can find flimsier ones online that deform under a screwdriver. This sits comfortably in the reliable middle, and in my installs it sealed up easily and stayed put.

If your application calls for a wider round (6" or 8"), go with the appropriately sized transition rather than stepping down to 4". It’ll be quieter, your fan will breathe better, and you won’t be fighting backpressure.

Bottom line

The Rocaris rectangular-to-round adapter is a straightforward, well-made fitting that solves a common retrofit problem without fuss. It’s compact, resists corrosion, and has a smooth interior that avoids the airflow penalties of rougher transitions. Installation is simple if you have basic HVAC hand tools and the right tape and screws on hand.

I recommend it for microwaves, bath fans, and other 4" round duct runs where you need to connect a 3-1/4" x 10" rectangular outlet in tight spaces. Measure your appliance’s opening, plan for a slip-in rectangular fit, and expect to supply your own fasteners and tape. If your hood needs a larger round, choose a larger transition—no adapter makes up for undersized duct. Used within its lane, this little fitting works like it should and holds up, which is exactly what I want from a piece of sheet metal buried behind cabinets.