One jin is exactly 0.5 kilograms — half a kilo, 500 grams. The modern Chinese market catty was deliberately redefined to that round metric value, which is why it survived metrication so comfortably.
About the units
The jin, written 斤 and often called the catty in English, is a traditional East Asian mass unit with a history reaching back more than two thousand years. Its value varied across dynasties and regions, generally falling somewhere between 500 and 600 grams. When mainland China standardised weights and measures in 1959, the jin was fixed at exactly 500 g, making it half a kilogram and trivially convertible. It is subdivided into ten liang, so one liang is 50 g — the older system used sixteen liang to the jin, which is the origin of the Chinese idiom about being "half a jin against eight liang", meaning six of one and half a dozen of the other. Taiwan and Hong Kong retain different values: the Taiwanese jin is 600 g and the Hong Kong catty is about 604.8 g.
The exact factor
In mainland China the factor is exactly 0.5, so jin to kilograms halves and kilograms to jin doubles. Both directions are exact. The regional variants are the complication: 1 Taiwanese jin is 0.6 kg and 1 Hong Kong catty is 0.60479 kg, so a recipe or market price from Taipei or Hong Kong is twenty per cent off if converted with the mainland factor. Since the unit is written with the same character in all three places, the source matters more than the notation.
Where you meet this conversion
Food shopping in mainland China is the everyday case. Market prices for vegetables, fruit, meat and fish are almost always quoted per jin rather than per kilogram, and street signage uses it even where packaged goods carry metric labels. Chinese recipes give quantities in jin and liang. Body weight is also commonly stated in jin in casual conversation, so someone describing themselves as 120 jin weighs 60 kg. Anyone reading Chinese sources or shopping in a Chinese market needs the halving reflex.