Electron-proton mass ratio

$mₑ/mₚ$ = 5.446 170 214 889(94) × 10⁻⁴

Value: 5.446 170 214 889 × 10⁻⁴
Standard uncertainty: 9.4 × 10⁻¹⁵
Relative uncertainty: 1.7 × 10⁻¹¹ (17 parts per trillion)
Status: Measured
Symbol: $mₑ/mₚ$
Source: CODATA 2022 via NIST · NIST entry

Use in a calculation

# electron-proton mass ratio, CODATA 2022 (NIST)
electron_proton_mass_ratio = 5.446170214889e-4  # standard uncertainty 9.4e-15

# SciPy: scipy.constants.physical_constants['electron-proton mass ratio']

What it is

The mass of the electron divided by the mass of the proton. Ratios like this are often measured more precisely than either quantity on its own, which is why CODATA lists them.

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	5.446 170 214 889(94) × 10⁻⁴	1.7 × 10⁻¹¹	+0.1 σ of 2018
2018	5.446 170 214 87(33) × 10⁻⁴	6.1 × 10⁻¹¹	+2.6 σ of 2014
2014	5.446 170 213 52(52) × 10⁻⁴	9.5 × 10⁻¹¹	−1.9 σ of 2010
2010	5.446 170 2178(22) × 10⁻⁴	4.0 × 10⁻¹⁰	+0.0 σ of 2006
2006	5.446 170 2177(24) × 10⁻⁴	4.4 × 10⁻¹⁰	+0.2 σ of 2002
2002	5.446 170 2173(25) × 10⁻⁴	4.6 × 10⁻¹⁰	−1.2 σ of 1998
1998	5.446 170 232(12) × 10⁻⁴	2.2 × 10⁻⁹

“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).