Page "Comparison of American and British English" Paragraph 21

* Full stops / Periods in abbreviations: Americans tend to write Mr., Mrs., St., Dr .; the British will most often write Mr, Mrs, St, Dr, following the rule that a full stop / period is used only when the last letter of the abbreviation is not the last letter of the complete word.

This kind of abbreviation is known as a contraction in the UK.

The use of full stops / periods after most abbreviations can also be found in the UK, although publications generally tend to eschew the use of American punctuation.

Unit symbols such as kg and Hz are never punctuated.