Length

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 $\sqrt{\frac{\hbar G}{c^3}}$.

In Stoney units, length is measured in Stoney lengths, defined as the ratio $\sqrt{\frac{G k_e e^2}{c^4}}$.

In atomic units, length is measured in atomic lengths, defined as the ratio $\frac{\hbar^2}{m_e k_e e^2}$.

In quantum chromodynamics units, length is measured in quantum chromodynamics lengths, defined as the ratio $\frac{\hbar}{m_p c}$.