List of largest libraries in the United States

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The size of libraries in the United States is determined by a number of metrics, including number of holdings (in terms of volumes or titles held), by circulation (i.e., library materials checked out or renewed); or by number of library visits.^[1]

The largest public library in the United States and the second largest library in the world is the Library of Congress in Washington, D.C., which is the de facto national library of the United States.^[2] It holds more than 167 million items, including "more than 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."^[3] The largest research library in the United States is the Harvard Library in Cambridge, Massachusetts.

Comparing the size of public libraries with research libraries (such as academic libraries) is complicated by the different definition 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."^[4] In contrast, the Public Library Data Service Statistical Report (a publication of the Public Library Association, which is 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."^[4]