Japanese 45 cm torpedo

Numerous 45 cm (17.7-inch or 18-inch) torpedoes have been used by the Imperial Japanese Navy and Imperial Japanese Navy Air Service. The size category has not been used by Japan since the end of the Second World War.

Torpedoes of 18-inch caliber were much more common during the early 20th century. Historically, this size category was often used by aircraft. Submarines and surface ships generally used 53 cm torpedoes, and surface ships additionally used 61 cm torpedoes. Japan also employs 32 cm torpedoes which conform to the NATO 12.75-inch (323.8mm) standard originally defined by the dimensions of the Mark 46 torpedo; these are dedicated ASW weapons, often delivered via aircraft. Due to their historical importance, this page also includes the smaller 14-inch (35.6cm) predecessors to the 45 cm torpedoes, and some of the miscellaneous developments during the Second World War.

Up to the beginning of the First World War, Japan had used more torpedoes against live targets than all the other navies of the world combined. During that time period, the torpedoes listed in this article were the primary ones employed.

Prior to 6 October 1917, imperial measurements were used. After this date, metric units were used. As such, the 18-inch torpedoes were designated as 45 cm torpedoes. Japanese torpedoes have usually conformed to the 45 cm (17.7-inch or 18-inch), the 53 cm (21-inch), and the 61 cm (24-inch) calibers.

The Japanese type designation scheme has mostly used three different approaches. Units designed prior to the end of the Second World War relied on the traditional Japanese calendar and were designated by either the regnal era year or the imperial year. In 1873, the Gregorian calendar was introduced in Japan; during the latter half of the 20th century, Japan increasingly switched to using this system, and as such, more recent torpedoes have type designations denoting Gregorian years. As an example of all three systems, a torpedo designed or accepted for service in 1980 could potentially be called either a Type 55 (Showa Era year 55), a Type 40 (Imperial Year 2640), or a Type 80 (Gregorian year 1980).