the si unit of length is the meter which is defined as the length equal to
Metre The metre (British spelling) or meter (American spelling) (from the Greek noun μέτρον, "measure") is the base unit of length in the International System of Units (SI). The SI unit symbol is m.[1] The metre is defined as the length of the path travelled by light in a vacuum in 1/299 792 458 seconds.[2]