The API Scale was selected in 1921 by the American Petroleum Institute (API), the U.S. Bureau of Mines, and the National Institute of Standards and Technology (NIST) as the standard hydrometer scale for petroleum products. The API Scale is based on the formula below.

General Specifications for Bellwether ASTM Hydrometers: Manufactured in strict accordance to specifications of ASTM (American Society for Testing and Materials) for accuracy and design. All instruments are suitable for certification for use as a primary reference standard with NIST traceability.