Pressure is a measure of the force against a surface. Pressure is usually expressed as a force per unit area. Additional Info: Many of the items you will find on an MSDS come in both English (U.S. Customary System) and metric (International System or SI or cgs) units. The metric system has been adopted by almost every country except the United States. Even in the U.S., scientists and technical people use the metric system because of its ease of use. If you are unfamiliar with the terms used in these definitions, See: Mass Unit and Distance Unit Conversion pages. MSDS Relevance: Be very careful to note the units when reading numbers on an MSDS. If you ever perform a calculation of any sort, always remember to write the units next to each number in your calculation and make sure that they cancel properly. See also: Area Units, Distance Units, Mass Units, Energy Units, Mole, Temperature Units, Vapor Pressure, Volume Units