I have a main window that I center this way in main.cpp : int main(int argc, char *argv[]) { QApplication app(argc, argv); QMainWindow *qmain = new QMainWindow; Ui_MainWindow ui; ui.setupUi(qmain); QRect r = qmain->geometry(); r.moveCenter(QApplication::desktop()->availableGeometry().center()); qmain->setGeometry(r); qmain->show(); return app.exec(); } In this "qmain" main window, I can create another QMainWindow and I try to make stick (or glue I don't know how to say) the two windows. Actually, I would like the right-top corner of the first one to be located at the left-top of the second one. For this, I use the following Ui_MainWindow's member function : void Ui_MainWindow::generate_IC() { qmenu = new QMainWindow; DiskMenu = new Ui_DiskGalMenu; DiskMenu->setupUi(qmenu); setInitialDiskMenuPosition(qmenu, this); qmenu->show(); } and the setInitialDiskMenuPosition : void Ui_MainWindow::setInitialDiskMenuPosition(QMainWindow *MainWindow, Ui_MainWindow *parent) { QSize size = parent->widget->size(); QDesktopWidget* desktop = QApplication::desktop(); int width = desktop->width(); int height = desktop->height(); int mw = size.width(); int mh = size.height(); int right = (width/2) + (mw/2); int top = (height/2) - (mh/2); MainWindow->move(right, top); } But the problem is that I don't get exactly what I want since the centralwidget size "parent- widget-size()" only returns the size of the widget and not the whole "qmain" parent window. So I have a light shift because the borders of the "qmain" window are not taken in account like it is shown below : If I could have access to the size of the whole parent window... If anyone could help me