Writing Numbers in English (I)

Today I would like to talk about good practices for writing numbers in English.


In science, there are two basic rules for writing whole numbers comprising four or more digits:


  • If the number has exactly four digits (not counting decimals), all the digits must be written together (e.g., 1234, NOT *1,234*).

  • For numbers of five or more digits (not counting decimals), a thin space must be used to separate every 3 digits starting from the right-most integer (e.g., 12 345; 123 345; 1 234 567; NOT *12,345*; *123,456*, *1,234,567*)


According to the International System of Units, periods and commas can only separate the integer and decimal parts of numbers. Choosing one or the other will depend on the country — not the language.


Therefore, commas must not be used to separate thousands in English.



Using a thin space to separate thousands has been officially endorsed by the:


  • ISO 80000-1:2009 standard

  • International Bureau of Weights and Measures

  • International Union of Pure and Applied Chemistry

  • American Medical Association Manual Style


I will discuss more good practices to write numbers in English in future posts, so stay tuned for more!

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