19" Racks have been metricated for donkey's years.
Width of faceplate = 19" = 482.6mm rounded to 483mm
Gap between rack strips (width of opening) = 455mm (typical)
Width of box = 450mm (typical)
Height of faceplate (1U) = 1.75" = 44.45mm rounded to 44.5mm
Height of box = 43mm (i.e. typically slightly less than height of faceplate)
Overall size of conventional floor-standing cabinet typically 600, 800,
900mm wide and typically 600, 700, 800, 900mm deep, etc.