How to get AppBar height in Flutter

This is not an ideal way, I think, but it will work.

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;

You can use this height to reduce from the screen height.

final double height = MediaQuery.of(context).size.height;