533millimeter

533 millimeter (533 mm) refers to a metric unit of length equal to 0.533 meters. It is commonly used as a designation for the diameter of submarine torpedo tubes and the torpedoes that fit them. In the imperial system, the same size is widely known as 21 inches, making 533 mm roughly equivalent to 20.99 inches.

The 533 mm diameter has become a standard in modern naval warfare. It is used by many navies around the world, notably in Russian and Soviet designs, and by Western forces that operate 21-inch torpedoes. The standardization permits interchangeability among launch systems and a broad family of torpedoes designed to fit a 533 mm tube.

Torpedoes of this diameter are produced in various weights and guidance types, for both anti-ship and anti-submarine roles. Submarines and some surface combatants employ 533 mm tubes to carry and launch these weapons, often in multiple loading bays for reduced reload times. The dimension continues to be the dominant torpedo diameter in many fleets, while some operators maintain compatibility with alternative sizes due to refits or legacy systems.

See also: 21-inch torpedo; Torpedo tube