The king cobra averages at 3 to 4 m (9.8 to 13.1 ft) in length and typically weighs about 6 kg (13 lb). The longest known specimen was kept captive at the London Zoo, and grew to around 18.5 to 18.8 ft (5.6 to 5.7 m) before being euthanised upon the outbreak of World War II. The heaviest wild specimen was caught at Royal Island Club in Singapore in 1951, which weighed 12 kg (26 lb) and measured 4.8 m (15.7 ft), though an even heavier captive specimen was kept at New York Zoological Park and was measured as 12.7 kg (28 lb) at 4.4 m (14.4 ft) long in 1972.
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King cobras are sexually dimorphic in size, with males reaching larger sizes than females. The length and mass of the snakes highly depend on their localities and some other factors. Despite their large sizes, typical king cobras are fast and agile. Some viper species, such as the eastern diamondback rattlesnake and the Gaboon viper, often much shorter in length but bulkier in build, rival the king cobra in average weight and reportedly best them in maximum weight.