**Length** is a physical quantity that gives the measure of something in one-dimensional space, and is typically classed as one of the fundamental units along with mass, time, current, temperature, and luminous intensity. The total length when considering the size of a shape is called edge length, and the vector quantity with dimensions of length used when considering physical distances is called displacement.

## Units[]

In SI units, length is measured in metres.

In Planck units, length is measured in Planck lengths, defined as the ratio .

In Stoney units, length is measured in Stoney lengths, defined as the ratio .

In atomic units, length is measured in atomic lengths, defined as the ratio .

In quantum chromodynamics units, length is measured in quantum chromodynamics lengths, defined as the ratio .