The default scene-view scale is 1:1 thus one logical unit in scene coordinates corresponds to one pixel in the view coordinates. If you want to change that, you need to "zoom" the view on the scene using QGraphicsView::scale() or similar. It's similar between the scene and the item. If you want an item 100cm wide and the scene is scaled in metres then you need to scale down the item 100 times so that 1 logical unit of item coordinates becomes 1/100 logical units of scene coordinates. If I'm not clear enough then feel free to ask for further explanations.