ConstantID

Unified atomic mass unit

UnitsMeasured

u = 1.660 539 068 92(52) × 10−27 kg

Value
1.660 539 068 92 × 10−27 kg
Standard uncertainty
5.2 × 10−37 kg
Relative uncertainty
3.1 × 10−10 (313 parts per trillion)
Status
Measured
Symbol
u
Source
CODATA 2022 via NIST · NIST entry

Use in a calculation

# unified atomic mass unit, CODATA 2022 (NIST)
unified_atomic_mass_unit = 1.66053906892e-27  # kg, standard uncertainty 5.2e-37

# SciPy: scipy.constants.physical_constants['unified atomic mass unit']

What it is

The dalton (Da): one twelfth of the mass of a carbon-12 atom, the unit for atomic and molecular masses. It has the same value as the atomic mass constant.

In other units

UnitValue
CGS (g)1.660 539 068 92 × 10−24
u dalton / atomic mass unit1unit is itself measured
MeV/c² MeV / c²931.494 103 719
GeV/c² GeV / c²0.931 494 103 719

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).

unified atomic mass unit: CODATA values 1998–2022 relative to the 2022 value, in parts per billion, with one-standard-uncertainty bars−300−200−1000+100+200+300parts per billion1998200220062010201420182022

Relative uncertainty

unified atomic mass unit: relative standard uncertainty in each CODATA adjustment, logarithmic scale10⁻¹⁰10⁻⁹10⁻⁸10⁻⁷10⁻⁶1998200220062010201420182022
CODATAValue (uncertainty)Relative uncertaintyChange from previous
20221.660 539 068 92(52) × 10−273.1 × 10−10 +4.6 σ of 2018
20181.660 539 066 60(50) × 10−273.0 × 10−10 +1.3 σ of 2014
20141.660 539 040(20) × 10−271.2 × 10−8 +1.6 σ of 2010
20101.660 538 921(73) × 10−274.4 × 10−8 +1.7 σ of 2006
20061.660 538 782(83) × 10−275.0 × 10−8 −0.3 σ of 2002
20021.660 538 86(28) × 10−271.7 × 10−7 +1.0 σ of 1998
19981.660 538 73(13) × 10−277.8 × 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).