Atomic mass constant

In physics, the atomic mass constant m_u is 1/12 of the mass of the ¹²C nuclide, in its ground state and at rest.

$m_{\mathrm {u} }={\frac {1}{12}}m(^{12}C).$

By definition m_u is equal to 1 u the unified atomic mass unit. The latest value is m_u = 1.660 538 782 × 10⁻²⁷ kg.