How to Measure for a Square-to-Round Package Unit Adapter

A square-to-round package-unit adapter must match both sides of the connection: the rectangular equipment or curb opening and the round duct collar. Taking measurements in a consistent order helps prevent a rotated, undersized, or mismatched adapter.

1. Identify each opening

Determine which rectangular opening is supply and which is return. If the application uses a two-piece adapter set, keep the measurements for the two openings separate. Label photos and notes so the supply dimensions are not accidentally assigned to the return adapter.

2. Measure the rectangular opening

Measure the clear opening width and length at the connection point. Record dimensions in the same order used by the product variant. Measure the actual opening rather than the outside of a nearby cabinet or curb unless the drawing specifically calls for outside dimensions.

Check the opening for corner radius, lips, fasteners, gaskets, or obstructions that could affect the connection. If an existing adapter is being replaced, confirm that the old adapter matches the equipment opening instead of assuming its dimensions are correct.

3. Confirm the round collar diameter

Measure or verify the required round duct diameter for each adapter. The collar diameter must match the planned duct connection. If the duct design calls for a transition elsewhere, do not substitute a different collar size without checking the airflow design.

4. Verify orientation

Note which rectangular side faces the front, rear, left, and right of the unit. Photograph the opening from above and mark the service-panel side. This is especially important when the rectangular opening is not square or when a pair of adapters must fit side by side.

5. Check adapter depth and clearance

Confirm the available vertical space between the package unit and the round duct connection. Allow for insulation, seams, fasteners, and access to adjoining components. Review the product specification drawing for the selected size.

6. Use the equipment model as supporting information

The manufacturer and model number can help identify the application, but field measurements should still be verified. Curbs, replacement units, and existing ductwork can change the final connection dimensions.

Measurement checklist

• Supply rectangular opening: width × length
• Return rectangular opening: width × length
• Required round collar diameter for each opening
• Single adapter or two-piece set
• Unit orientation and service-panel side
• Available adapter depth and surrounding clearance
• Equipment manufacturer and model number

View our flat insulated square-to-round package-unit adapters. Before ordering a complex configuration, ask a product question and attach your measurements, model number, and clear photos.