The fictional platform number in J.K. Rowling’s Harry Potter books is Platform 9¾.
The platform is the hidden departure point for the Hogwarts Express at London’s King’s Cross Station. Young witches and wizards reach it by passing through the barrier between the real platforms 9 and 10, a magical entrance that ordinary travelers cannot see.
The Hogwarts Express departs on 1 September each year, carrying students to Hogsmeade Station near Hogwarts. The platform therefore becomes a recurring symbol of transition: characters leave the ordinary Muggle world and begin another school year in the wizarding world.
The number is often written with the fraction symbol “¾,” although “9 3/4” is also used in ordinary text. King’s Cross later installed a real Platform 9¾ sign for visitors, but the fictional platform does not correspond to an actual railway platform.