Milligram (mg) is a unit of weight or mass in the International System of Units (SI) . It is equal to one thousandth of a gram, which is the base unit for mass in the metric system. The prefix "milli" means one thousandth, so there are 1,000 milligrams in a gram. The abbreviation or symbol for milligram is "mg". Milligram is commonly used to measure small quantities of substances, such as powdered chemicals.