The names of metric units are usually spelled out.

• A 20-kilogram sack, not a 20-kg sack
• covering 50 hectares, not covering 50 ha
• ran 10 kilometres, not ran 10 km
• the room was 900 square metres, not the room was 900 sq/m.

There are no abbreviations in metric, only symbols. These symbols take periods only at the end of a sentence, never take an s in the plural and are separated from a number by a space. In adjectival uses, the space is replaced by a hyphen.

C (for Celsius) can be used on first reference. Other common symbols such as km/h and mm may be used on second reference when preceded by a number: at 70 km/h, a 105-mm cannon.