There is no ‘rule’ about when to use unit symbols (eg kg) or words (eg kilogram) for units. Here are some guidelines:

• If few units are used in the text, use words.
• Use symbols in tables and figures if space is limited.
• If the document uses many units
  • write in full at first use and give the symbol in brackets; then use symbols from then on, or
  • use symbols and provide a table explaining them.
• If uncertain, use words for units when the document is intended for a general audience, particularly if it contains few units.