公斤(gōng jīn) - kilogram in Mandarin

公斤 (gōng jīn) is the standard metric kilogram, built from 公 (gōng, "public, metric") plus the older weight unit 斤 (jīn). Putting 公 (gōng) in front of a traditional unit is the regular pattern Mandarin uses to build metric equivalents, the same trick behind 公里 (gōng lǐ, "kilometre") and 公分 (gōng fēn, "centimetre").

As a measure word it goes straight after the number: 十公斤 (shí gōng jīn), "ten kilograms", with no particle in between. You will meet 公斤 (gōng jīn) on official documents, in science and health contexts, and on packaging, while the market stall down the street is still more likely to quote a price in 斤 (jīn) - remember that 一公斤 (yī gōng jīn) is double 一斤 (yī jīn), not the same thing.