List of largest libraries in the United States

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] 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.

ARL uses the ANSI/NISO Z39.7-2004 definition of "volume": "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 catalogued, classified, and made ready for use.

A book or serial title may be distinguished from other titles by its unique International Standard Book Number (ISBN) or International Standard Serial Number (ISSN).

"[8] Multiple copies of the same work (for example, subscriptions to the same publication) are counted as a single title, but a serial title available in multiple formats (for example, print and online) are counted once for each available format.

The Thomas Jefferson Building at the Library of Congress in Washington, D.C. , the largest library in the United States and second-largest library in the world with over 167 million holdings, including 39 million books and other printed recordings, 14.8 million photographs, 5.5 million maps, 8.1 million pieces of sheet music, and 72 million manuscripts
The Main Branch at New York Public Library in New York City , the nation's largest public library with over 25 million holdings
Widener Library , part of the Harvard Library at Harvard University , the largest research library in the world and nation with nearly 20 million holdings