Muon mass in u

$m_μ$ = 0.113 428 9257(25) u

Value: 0.113 428 9257 u
Standard uncertainty: 2.5 × 10⁻⁹ u
Relative uncertainty: 2.2 × 10⁻⁸ (22 parts per billion)
Status: Measured
Symbol: $m_μ$
Source: CODATA 2022 via NIST · NIST entry

Use in a calculation

# muon mass in u, CODATA 2022 (NIST)
muon_mass_in_u = 1.134289257e-1  # u, standard uncertainty 2.5e-9

# SciPy: scipy.constants.physical_constants['muon mass in u']

What it is

The rest mass of the muon, an unstable heavier cousin of the electron (about 207 times its mass) that lives about 2.2 microseconds, in unified atomic mass units (daltons), where 1 u is one twelfth of the mass of a carbon-12 atom.

In other units

Unit	Value
CGS (g)	1.883 531 627 × 10⁻²⁵
MeV/c² MeV / c²	105.658 3755
GeV/c² GeV / c²	0.105 658 3755

Converted with ConstantID's unit table; digits are rounded to the precision of the value, and a long exact value is rounded only for display. The relative uncertainty is unchanged by conversion to an exactly defined unit.

How the value changed, 1998–2022

Each point is one CODATA adjustment, plotted as its difference from the 2022 value; the bars show one standard uncertainty (an arrowhead means the bar runs off the chart).

Relative uncertainty

CODATA	Value (uncertainty)	Relative uncertainty	Change from previous
2022	0.113 428 9257(25)	2.2 × 10⁻⁸	−0.1 σ of 2018
2018	0.113 428 9259(25)	2.2 × 10⁻⁸	+0.1 σ of 2014
2014	0.113 428 9257(25)	2.2 × 10⁻⁸	−0.3 σ of 2010
2010	0.113 428 9267(29)	2.6 × 10⁻⁸	+0.4 σ of 2006
2006	0.113 428 9256(29)	2.6 × 10⁻⁸	−0.3 σ of 2002
2002	0.113 428 9264(30)	2.6 × 10⁻⁸	+2.8 σ of 1998
1998	0.113 428 9168(34)	3.0 × 10⁻⁸

“Change from previous” is the shift in the value divided by the standard uncertainty of the previous adjustment. Earlier values are converted into the current unit where NIST changed it (for example J s to J Hz⁻¹).