Half. His father is fully ethnically German, as in his father is the son of ethnic German parents who immigrated to the United States. Trump’s mother, on the other hand, was born in Scotland to a Scottish family. So he is 50% German and 50% Scottish.
It kind of makes this whole “ending birthright citizenship” thing look awful — since this man’s family has been in America for only two generations on his father’s side, and for only one on his mother’s side. Mary Trump was born in the village of Tong, on the Isle of Lewis in Scotland. She was just about as Scottish as they come, and spoke for all her life with a noticeable accent. Trump’s paternal grandfather died before he was born, but he lived long enough to know his paternal grandmother, a woman who never lost her strong German accent and continued to be most fluent in her native German rather than English.
In a way, it is very telling of what America is, as a country — that the most “American” man you can think of, the McDonald’s loving, celebrity-obsessed, vain, self-aggrandizing lover of big buildings, fast cars and beautiful women, the celebrity-turned-POTUS, is the direct product of very-recent immigration.