The general purpose of this publication is to encourage and facilitate uniformity of metric practice within the petroleum industry. The specific purposes are as follows:- To define metric practice for the petroleum industry;- To encourage uniformity of metric practice and nomenclature within the petroleum industry and- To facilitate the use of SI in all aspects of the petroleum industry. Use of this publication by the American Petroleum Institute, its divisions, and its members implements API's policy and also implements recommendations in ISO 1000-1992, SI Units and Recommendations for the Use of Their Multiples and of Certain Other Units [I].This publication specifies the API preferred units for quantities involved in petroleum industry measurements and indicates factors for conversion of quantities expressed in customary units to the API preferred metric units. The quantities that comprise the tables are grouped into convenient categories related to their use. They were chosen to meet the needs of the many and varied aspects of the petroleum industry but also should be useful in other, similar process industries.