What is the SI unit for length?

The SI unit for length is the meter. The International System of Units (SI) was established to provide a standard measurement system that is universally recognized and used in scientific and technical contexts. The meter serves as the base unit for length and is defined based on physical constants, specifically the distance light travels in a vacuum in a specified fraction of a second.

Other options, such as centimeter, kilometer, and millimeter, are all metric units of length as well, but they are derived from the meter. A centimeter is one-hundredth of a meter, a kilometer is one thousand meters, and a millimeter is one-thousandth of a meter. While these units are useful for specific applications, the meter is considered the standard and fundamental unit in the SI system for expressing length.