This problem has existed in linguistics since at least the 1950s. Dictionary-reference book “Russian literary pronunciation and stress” 1959 indicatesthat shape kilograms used primarily in written language kilogram – orally.

This division lasted until the early 2000s. And some sources recognized option kilogram non-normative, while others considered it acceptable.

Modern dictionaries indicatethat both options are equal, but with one caveat: if the noun is used with a numeral. This is true called counting form, which is used when specifying the exact amount: nine kilograms and nine kilos.

However, outside of such combinations with numerals, one should speak and write only a word with the ending -ov: missing a few kilograms, counting kilograms.