Check out example codes for "how to subtract he height of appbar in flutter". It will help you in understanding the concepts better.

Code Example 1

Firstly declare the AppBar widget that you will use in your Scaffold.

Widget demoPage() {
  AppBar appBar = AppBar(
    title: Text('Demo'),
  );
  return Scaffold(
    appBar: appBar,
    body: /*
    page body
    */,
  );
}
Now you can get the height of your appBar using its preferredSized:

double height = appBar.preferredSize.height

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