Add QPushButton to main widget in designer and press Ctrl+G.
It’s the simplest way.

Sorry, this does not solve anything

it just stretches the button, so its as wide as mainwindow, and the button´s geometry is disabled, so i cant move it any further

But now it’s…

andyc wrote:

in the centre of the window even if the window is resized

See How to center a window on the screen [developer.qt.nokia.com]
But instead of desktop available geometry, use button parent geometry.

andyc wrote:

Hello, how i can i allign my pushbutton in the centre of the mainwindow ? ive been googling for a while, and didnt find any answer…
also, i would like to have my button in the centre of the window even if the window is resized…

Best regards,
OSiRiS

Do you mean like using a layout? Otherwise, can you elaborate?