So Wikipedia, here, citing Census data:
German Americans are citizens of the United States of German ancestry and comprise about 51 million people, or 17% of the U.S. population, the country's largest self-reported ancestral group. California, Texas and Pennsylvania have the largest numbers of German origin, although upper Midwestern states, including Ohio, Iowa, Minnesota, Wisconsin, Nebraska, and the Dakotas, have the highest proportion of German Americans at over one-third.