When is sir capitalized is a tricky question. It largely depends on the way that you use that word.
When Sir Is Capitalized
There are only two cases when the word ”sir” is capitalized and those are
- When it is used as the first word of a sentence or salutation.
- When one uses it as an honorific, for example for a person who has been knighted.
“I and my family were there when Sir Alex Ferguson coached his final match for Manchester United.”
“Sir, I am writing to you to remind you of your one o’clock appointment tomorrow.”
*Have you seen that new movie starring Sir Ian McKellen?”
There is a bit of a tricky exception to this rule. If you are calling the person who has deserved the honorific sir by their full name than sir must go before their name and be capitalized. If you are only referring to them by the word sir and that world stands alone it will not be capitalized.
“My grandfather is an Aberdeen fan and he has seen Sir Alex Ferguson coach in the early days, long before anyone thought that he would one day be referred to as sir.”
When Sir Is Lowercase
When you use sir as an honorific in a new conversation, it has to be lowercase. A lot of writers who write dialogue struggle with this.
“It was a pleasure to talk this over with you, sir.”
“Funny crashing into you here sir.”
“Have you any interest in planes, sir?”
You need to capitalize sir when you are starting a letter or email. You also need to capitalize sir if you are using it as an honorific before the person’s name. In every other case, sir should be lower case.