The SI unit for length is the meter (m) . It is defined as "the length of the path traveled by light in vacuum within 1/299792458 seconds". The meter is the base unit of length in the International System of Units (SI) . Other units of length can be derived from the meter by adding prefixes, such as millimeter or kilometer, to produce systematic decimal multiples and submultiples of the base unit that span many orders of magnitude. In the U.S. customary system, the basic unit of length is the yard, and other units include the inch, foot, and mile.