Как правильно –
Half a kilo
Или
Half OF a kilo
Давайте разбираться.
На самом деле, в большинстве случаев эти конструкции взаимозаменяемы.
Half a million US children have been diagnosed with Covid-19 (CNN)
Antibody tests used to determine if people have been infected in the past with Covid-19 might be wrong up to half the time, the US Centers for Disease Control and Prevention said in new guidance posted on its website. (CNN)
Sweden recorded its highest death tally in 150 years for the first half of 2020, according to the country’s official statistics office. (CNN)
ОДНАКО:
Мы используем half of – перед местоимениями
CNN Loses Half Of Its Audience To The Interwebs (nyuocal.com)
А вот перед такими словами как hour, kilo, metre etc следует использовать просто half
It took us half an hour to get to the Central Bus Station