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!