A range hood duct transition should match the new hood’s approved outlet and minimum duct size. The old cabinet hole does not set the right part.
If the manual calls for 6-inch or larger round metal duct, don’t squeeze it into a smaller legacy run because a reducer happens to fit.
Start with the manual.
Then measure both duct ends, the centerline offset, and the clear height above the hood. A transition is the right buy only when the damper can open freely and the full route stays within that model’s vent limits.
The home installation guide gives you a simple place to record cabinet, framing, and access measurements before anything is cut.
Range hood duct transition choices begin with the model
Find the exact model number and download its installation guide. Retail listings often say “6-inch duct” or “convertible,” but they may not show the permitted outlet direction, minimum diameter, supplied damper, transition type, or maximum equivalent length.
One Whirlpool range-hood manual, for example, permits either 3¼-by-10-inch rectangular metal vent or 6-inch-or-larger round metal vent. It also calls for a 3¼-by-10-to-6-inch-or-larger transition when the round route is used.
That is a model-specific approval, not a universal size chart.
The same manual recommends rigid metal, says not to use a smaller duct than specified, and requires the route to end outdoors. Those details matter more than a fitting’s product title because the transition cannot repair an undersized, flexible, or attic-terminated run.
A Broan-NuTone installation guide makes the fit check equally concrete: use the transition for a 6-inch round duct, seal the joint with metal foil tape, and confirm that the adapter enters the duct while the damper opens freely.
Put six measurements on one card
Measure before you order.
| Record | What to copy or measure | Why it changes the purchase |
|---|---|---|
| Hood outlet | Shape, width, height or diameter, and top or rear discharge | Sets the transition’s starting end |
| Manual minimum | Approved material and smallest permitted duct | Stops a convenient but undersized reducer |
| Existing duct | Inside shape, dimensions, material, and direction | Sets the far end and exposes flex or damaged duct |
| Clear height | Top of hood outlet to the first fixed duct edge | Rules out a transition that is too tall |
| Offset | Horizontal distance between both centerlines | Shows whether a straight fitting can align |
| Whole route | Straight length, elbows, cap, and termination | Tests the manual’s equivalent-length limit |
Take the hood dimensions from the manual, then verify the house duct with a tape measure. Don’t infer a duct’s size from the cabinet cutout; the opening can be much wider than the metal duct running through it.
Photograph both ends with the tape visible.
This measurement-first habit also prevents expensive fit errors outside the kitchen. The same idea appears in the smart mortise-lock fit check, where the visible product name matters less than the hidden opening and hardware dimensions.
Area and equivalent length catch hidden reductions
Shape labels alone hide a large change. An 8-inch round duct has about 50.3 square inches of area, while a 6-inch round duct has about 28.3.
Reducing a hood that requires 8 inches into an existing 6-inch run removes about 44 percent of that area. A shiny adapter can make the connection look finished while the motor sees a route the manufacturer never approved.
Area is only a warning screen; it doesn’t replace the manual. A 3¼-by-10-inch rectangle has 32.5 square inches, yet the approved round equivalent depends on the hood’s tested fittings, blower, route, and damper.
Route resistance needs its own calculation.
In the Whirlpool example above, a rectangular-to-round transition counts as 4.5 equivalent feet and each 90-degree round elbow counts as 5 feet. A route with that transition, two elbows, and 8 feet of straight duct therefore counts as 22.5 feet before the cap is considered.
That model allows up to 50 feet for its round system, but another hood may use different fitting penalties or a different maximum. Copy the numbers from your manual rather than borrowing this total.
A short cabinet can defeat the right fitting
One range-hood installation thread describes the block in plain words: tight cabinet space, and two openings that sit almost side by side so the shapes won’t line up.
The offset is the problem there, not the fitting.
Dry-fit the hood and transition before cutting the cabinet. Check the transition’s actual height, the round collar’s centerline, the damper swing, the nearest shelf or cabinet rail, and enough access to seal the full joint.
Don’t solve a two-inch offset by forcing flexible foil duct into an S-curve. It adds resistance, collects grease in its ridges, and conflicts with manuals that call for rigid metal.
A longer tapered transition or a custom offset fitting from a sheet-metal shop is often cleaner than stacking a transition and two tight elbows. If the fixed duct sits behind a stud, enters a shared shaft, or cannot be inspected to its outdoor cap, stop treating this as a fitting-only job.
Keep electrical work out of this measurement check. Don’t disconnect the hood or move its power supply as part of this inspection.
Cutting hidden building surfaces or changing a shared or fire-rated duct belongs with qualified local trades. They can also check permits and makeup-air rules.
Send a measured scope to the sheet-metal shop
You don’t need a long story to get a useful quote. Send the exact hood model, a photo of its outlet page, the existing duct dimensions and material, plus the clear height and centerline offset.
Add the route sketch with every elbow and the exterior cap.
Ask for two prices: an approved transition into the existing route, and the smallest reroute that preserves the manual’s minimum duct size. Require free damper movement, rigid metal, accessible sealed joints, and an outdoor termination in the written scope.
Reject a proposal that names only a reducer size. It hasn’t answered whether the new hood can move air through the route or whether the fitting physically fits above it.
Today, copy those six measurements beside the model’s vent limits before buying a range hood duct transition. That one card tells you whether an off-the-shelf part is enough or a custom fitting is the cheaper first purchase.
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