The picometre (International spelling as used by the International Bureau of Weights and Measures; SI symbol: pm) or picometer (American spelling) is a unit of length in the metric system, equal to one trillionth (i.e., ¹/_{1,000,000,000,000}) of a metre, which is the SI base unit of length. It can be written in scientific notation as 1×10^-12 m, in engineering notation as , and is simply .