List of largest libraries in the United States

The largest libraries in the United States is determined by the number of volumes or titles held, by circulation, including library materials checked out or renewed, or by number of library visits. As of 2018, the largest public library in the United States and largest library in the world is the Library of Congress in Washington, D.C., the de factonational library of the United States, which holds over 167 million items, including over 39 million books and other printed materials, 3.6 million recordings, 14.8 million photographs, 5.5 million maps, 8.1 million pieces of sheet music, and 72 million manuscripts. The largest research library in the United States, with nearly 20 million holdings, is the Harvard Library at Harvard University in Cambridge, Massachusetts.

Comparing the size of public libraries with research libraries, such as academic libraries, is complicated by the differing definitions of holdings or volumes used. The Association of Research Libraries uses the National Information Standards Organization definition of volume, which is, "A single physical unit of any printed, typewritten, handwritten, mimeographed, or processed work, distinguished from other units by a separate binding, encasement, portfolio, or other clear distinction, which has been cataloged, classified, and made ready for use, and which is typically the unit used to charge circulation transactions." The Public Library Data Service Statistical Report, a publication of the Public Library Association, a division of the American Library Association, defines "holdings" as "the number of cataloged items (number of items, number of titles) plus paperbacks and videocassettes even if uncataloged."