'An hr' is correct or not grammatically?

Answer 1

If the 'hr' is referring to the word 'hour' then yes, it is grammatically correct. When deciding whether to use 'A' or 'An', it depends on the pronunciation, not the letter used. In the word 'hour' you do not pronounce the 'h', so it sounds like it starts with an 'o', so you would use 'an'.

The abbreviation 'an hr' is not grammatically correct. It should be written 'an hour/An hour'. Sometimes 1hr or 2hrs is okay depends on the situation.