number sign (#)

Always spell out number in number sign. Don't use pound sign.

Don't use the # symbol preceding a numeral. Spell out number instead. For example, use number 7, not #7. When space is limited, as in tables, it's OK to abbreviate number as No.

Global tip The abbreviation No. isn't used to abbreviate number in other languages, so using No. for this purpose might lead to mistranslation in machine-translated content.

In social media, it's OK to use the # symbol to indicate a hashtag. Don't use number sign to refer to the hashtag.

Example
(on Twitter) A quick look at top conversations and #Office365 product announcements of the week.

It's OK to use the # symbol in references to the # key on the keyboard.

See also Keys and keyboard shortcuts term collection