(en) The NIST Guide to the SI: 9.3 Spelling unit names with prefixes reports that IEEE/ASTM SI 10-2002 states that there are three cases in which the final vowel of an SI prefix is commonly omitted: megohm, kilohm, and hectare. "In all other cases in which the unit name begins with a vowel, both the final vowel of the prefix and the vowel of the unit name are retained and both are pronounced." (pristupljeno 16. prosinca 2013.)
