Calculating the boundary box of a large number of polygons in a shapefile in QGIS is discussed earlier in this post. The process itself is tedious and required a support from Microsoft Office. There has been numerous updates arrived in QGIS and several algorithms were incorporated. Keeping this in mind I developed a QGIS model which can create an output with a simple click.

Download this Model and save it in suitable location.

More on this github repo

Open the shapefile in QGIS and follow these steps.

Now, select model tool box icon and select Open existing model. In the window that appears select above saved file and click apply.

After that in processing toolbox, we can see our Boundarybox model appears in Models tool.

Double click the Boundarybox icon and in the dialog box that appears unselect all the options denoting “Open output file after running algorithm” except for the last section [Bound (optional)] (see picture).

You can provide suitable name and save the file in the last option.

Click Run.

If you not saved the file in last option, then right click the temporary file named Bound (Default name) and save it in your required location.

• Check attribute table.

Hope it hepls.

Note: This model only works after QGIS 3.16