Thru is an American spelling of through. It is not standard. The COED says:
- prep., adv., & adj. chiefly US informal spelling of through.
According to Wiktionary:
- Thru is mostly used where the preposition through could be used (e.g. Monday thru Friday); it is less common as an adjective or adverb (I'm thru with the vacuuming). It is less used in formal situations, except in cases where brevity is wanted such as roadway signs or date ranges.
- This spelling is not used in standard British English, though it can be used as an abbreviation – similar to thro' that's still sometimes seen – and is recently becoming noticeable via American-based companies.
- It is often – in both American and British English – used in professional drafting (5/8″ thru hole) to save space and simplify annotation on a drawing for fabrication or construction.
As with other Americanisms it is simpler than the British spelling, e.g. :