I was curious about this question and in my search for an answer, I found this Williams Institute report, which appears to be the best summary of the various surveys and estimates out there. The other literature on this question pegs the percent of the adult population who identify as transgender between 0.1% and 0.5%. After reviewing this literature, the authors of this report average two prior surveys — one from California and one from Massachusetts — and end up estimating that around 0.3% of the US adult population identifies as transgender.
