Atomic mass constant (unified atomic mass unit)

m_{\mathrm{u}} = 1.66053906892 \times 10^{-27}\ \text{kg}

Value	1.66053906892e-27 kg
Status	Measured: ± 5.20e-37 kg (3.1e-10 relative)
Source	CODATA 2022
Categories	Universal & Atomic Chemistry atomic

m_u in every mass unit

microgram	1.6605391e-18 μg
milligram	1.6605391e-21 mg
gram	1.6605391e-24 g
kilogram	1.6605391e-27 kg
metric tonne	1.6605391e-30 t
ounce	5.8573792e-26 oz
pound	3.6608620e-27 lb
stone	2.6149014e-28 st
US short ton	1.8304310e-30 ton
grain	2.5626034e-23 gr
unified atomic mass unit (dalton)	1 u

Learning zone

Chemists and physicists spent decades using different atomic mass scales — oxygen-16 for physicists, natural oxygen for chemists — until the 1961 compromise on carbon-12, whose mass is exactly 12 u by definition. One dalton is 1.660 539 069 × 10⁻²⁷ kg, or 931.494 MeV/c² in energy terms, and its reciprocal is why a molar mass in g/mol and a molecular mass in daltons read as the same number.

The 2019 SI redefinition broke that identity at the tenth digit: with N_A now fixed by fiat, the molar mass constant M_u = N_A m_u is measured rather than exactly 1 g/mol, and m_u inherits a 3 × 10⁻¹⁰ relative uncertainty. The other trap is that atomic masses are not the sum of their parts — a ¹²C atom is lighter than six protons, six neutrons and six electrons by about 0.099 u, and that missing mass is precisely the nuclear binding energy.

Used by these solvers

Moles from Mass (n = m/M)

n = \frac{m}{M}