Q. Is a half sleeve jacket considered as formal?

Answer

Commonly known as a "short sleeve jacket" this garment is in no way appropriate for a formal occasion. Every man should have several button-down half-sleeve dress shirts in his closet. If someone dresses them up he/she may wear them to a variety of formal events and wear them to the office on a regular basis if they dress professionally. The jacket makes a difference. No it wouldn't be appropriate if the jacket had a button-down collar or a more formal cut. In contrast feel free to untuck a loose flowing blouse or something with a more casual vibe. However some should probably not wear it to crucial appointments.