Custom Size Duct Fittings: Order Any Nonstandard Dimension

The problem with 'just use the next size up'

When the exact size isn't on the shelf, the tempting fix is to grab the nearest standard fitting and make it work. That gap between nominal and actual gets closed with mastic, tape, or a stub of flex duct — and every one of those is a future leak, a pressure loss, or a callback.

A custom-size fitting eliminates the gap. The part is cut to the dimension you measured, so the connection is metal-to-metal at the right size the first time.

What 'custom size' actually covers

There is no fixed size menu. You set:

• Inlet dimensions — width × depth, to the inch or half-inch
• Outlet dimensions — for transitions, reducers and square-to-round
• Length or angle — straight length, elbow degree, taper length
• Throat radius — for elbows and offsets that must clear an obstruction
• Gauge and material — 26–20 gauge, galvanized, aluminum or stainless

If you can measure it, you can order it. Retrofit jobs where the existing duct never matched nominal sizes are exactly what custom-size ordering is for.

Measure twice — custom parts aren't returnable

The one discipline custom ordering demands: field-verify your dimensions before you submit. A part built to a wrong measurement is a part you paid to fabricate. Measure the actual duct or opening, not the drawing, and confirm inlet and outlet separately on transitions.

Every custom order shows a dimension drawing before checkout so you can catch a transposed width and depth before it becomes tin.

Standard sizes vs. what jobs actually need

Below is why the in-between sizes matter so often on real installs:

• A furnace plenum that measures 19½×19½, not a clean 20×20
• A joist bay that only clears an 8×8 when the run calls for 10×6
• A rooftop curb opening that is metric or field-cut and off-standard
• A retrofit tie-in to old duct that was hand-formed decades ago

Each of these is a two-minute custom order and a leak-free joint instead of a bodge.