List of the largest libraries in the United States

The American Library Association maintains a list of the 100 largest libraries in the United States by volumes held. The ALA's list was last updated in October 2012, although much of the most recent data is from 2009–2010 report by the Association of Research Libraries (academic institutions) and the 2011 Statistical Report of the Public Library Data Service.

A complication that arises when comparing the size of library collections is the different definition of holdings or volumes used by public libraries and academic/research libraries. 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." 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."

The largest public library is the Library of Congress in Washington, D.C. (the de facto national library of the United States), with 34,528,818 volumes. The largest academic library is the Harvard University Library, with 16,832,952 volumes. The largest library outside of the contiguous United States is the Hawaii State Public Library System, the statewide public library system of Hawaii, with 3,776,405 volumes, while the largest academic library outside the contiguous United States is the University of Hawaii at Manoa Library, with 3,602,058 volumes.

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