Neutron-proton mass difference in u

$mₙ − mₚ$ = 1.388 449 48(40) × 10⁻³ u

Value: 1.388 449 48 × 10⁻³ u
Standard uncertainty: 4 × 10⁻¹⁰ u
Relative uncertainty: 2.9 × 10⁻⁷ (288 parts per billion)
Status: Measured
Symbol: $mₙ − mₚ$
Source: CODATA 2022 via NIST · NIST entry

Use in a calculation

# neutron-proton mass difference in u, CODATA 2022 (NIST)
neutron_proton_mass_difference_in_u = 1.38844948e-3  # u, standard uncertainty 4e-10

# SciPy: scipy.constants.physical_constants['neutron-proton mass difference in u']

What it is

The neutron–proton mass difference in atomic mass units.

In other units

Unit	Value
CGS (g)	2.305 574 61 × 10⁻²⁷
eV/c² electronvolt / c²	1 293 332.5
MeV/c² MeV / c²	1.293 3325
GeV/c² GeV / c²	0.001 293 3325

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	1.388 449 48(40) × 10⁻³	2.9 × 10⁻⁷	+0.3 σ of 2018
2018	1.388 449 33(49) × 10⁻³	3.5 × 10⁻⁷

“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).