The term mass is often used interchangeably with "weight". Strictly speaking, this is incorrect because weight varies with the force of gravity and mass does not. For example, an astronaut would be almost weightless in outer space, but his or her mass would be unchanged from that on Earth. Therefore, one can say that mass is an expression of "how much stuff" something has. Here is a handy conversion calculator for some common and uncommon mass (weight) terms. Each term is defined in the next section. 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. 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, Mole, Pressure Units, Temperature Units, Volume Units