Some widely used pipe fitting standards are as follows:

ANSI: The American National Standards Institute

ANSI is a private, non-profit organization. Its main function is to administer and coordinate the U.S. voluntary standardization and conformity assessment system. It provides a forum for development of American national standards. ANSI assigns “schedule numbers”. These numbers classify wall thicknesses for different pressure uses.

ASME: American Society for Mechanical Engineers

This is one of the reputed organizations in the world developing codes and standards. The schedule number for pipe fitting starts from ASME/ANSI B16. The various classifications of ASME/ANSI B16 standards for different pipe fittings are as follows:

ASME/ANSI B16.1 – 1998 – Cast Iron Pipe Flanges and Flanged Fittings
ASME/ANSI B16.3 – 1998 – Malleable Iron Threaded Fittings
ASME/ANSI B16.4 – 1998 – Cast Iron Threaded Fittings
ASME/ANSI B16.5 – 1996 – Pipe Flanges and Flanged Fittings
ASME/ANSI B16.11 – 2001 – Forged Steel Fittings, Socket-Welding and Threaded
ASME/ANSI B16.14 – 1991 – Ferrous Pipe Plugs, Bushings and Locknuts with Pipe Threads
ASME/ANSI B16.15 – 1985 (R1994) – Cast Bronze Threaded Fittings
ASME/ANSI B16.25 – 1997 – Buttwelding Ends
ASME/ANSI B16.36 – 1996 – Orifice Flanges etc.

ASTM International: American Society for Testing and Materials

This is one of the largest voluntary standards development organizations in the world. It was originally known as the American Society for Testing and Materials (ASTM). This is a reputed scientific and technical organization that develops and publishes voluntary standards on the basis of materials, products, systems and services. This is a trusted name for standards. The standards covered by this organization covers various types of pipes, tubes and fittings, especially made of metal, for high-temperature service, ordinary use and special applications like fire protection. The ASTM standards are published in 16 sections consisting of 67 volumes.