I know this might sound obvious but the thickness of the finished strap depends on the thickness of the leather being used. In general I work with leather that will give a finished strap between 3mm and 5mm thick – in my opinion this is just about perfect.
If you want to know the thickness of a strap made from a specific leather you’ll need to go to that leathers page in my web store. Click on the photo of the leather in the straps page to see all information, including strap thickness, pricing and my suggestions for complimentary thread colours.