The unit abbreviation for inch is: in. (with a period). It is the only unit abbreviation that has a period at the end of it. Why? Well, some people say it’s there so people won’t confuse the inch unit (in.) with the word “in” when used in a sentence. Like that one. I like to think it’s a souvenir from the inchworm… As he crawls by, he leaves a little dropping at the end of his nickname…. … in. ….