ConstantID

Neutron-proton mass ratio

Atomic & nuclearMeasured

mn/mp = 1.001 378 419 46(40)

Value
1.001 378 419 46
Standard uncertainty
4 × 10−10
Relative uncertainty
4.0 × 10−10 (399 parts per trillion)
Status
Measured
Symbol
mn/mp
Source
CODATA 2022 via NIST · NIST entry

Use in a calculation

# neutron-proton mass ratio, CODATA 2022 (NIST)
neutron_proton_mass_ratio = 1.00137841946  # standard uncertainty 4e-10

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

What it is

The mass of the neutron 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).

neutron-proton mass ratio: CODATA values 1998–2022 relative to the 2022 value, in parts per trillion, with one-standard-uncertainty bars−1500−1000−5000+500+1000+1500parts per trillion1998200220062010201420182022

Relative uncertainty

neutron-proton mass ratio: relative standard uncertainty in each CODATA adjustment, logarithmic scale10⁻¹¹10⁻¹⁰10⁻⁹1998200220062010201420182022
CODATAValue (uncertainty)Relative uncertaintyChange from previous
20221.001 378 419 46(40)4.0 × 10−10 +0.3 σ of 2018
20181.001 378 419 31(49)4.9 × 10−10 +0.6 σ of 2014
20141.001 378 418 98(51)5.1 × 10−10 −0.4 σ of 2010
20101.001 378 419 17(45)4.5 × 10−10 −0.0 σ of 2006
20061.001 378 419 18(46)4.6 × 10−10 +0.8 σ of 2002
20021.001 378 418 70(58)5.8 × 10−10 −0.3 σ of 1998
19981.001 378 418 87(58)5.8 × 10−10

“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).

Related constants

Atomic & nuclear λC,n Neutron Compton wavelength 1.319 590 903 82(67) × 10−15 m Atomic & nuclear μn/μe Neutron-electron magnetic moment ratio 1.040 668 84(24) × 10−3 Atomic & nuclear mn/me Neutron-electron mass ratio 1838.683 662 00(74) Atomic & nuclear gn Neutron g factor −3.826 085 52(90) Atomic & nuclear γn Neutron gyromagnetic ratio 1.832 471 74(43) × 108 s−1 T−1 Atomic & nuclear γn/2π Neutron gyromagnetic ratio in MHz/T 29.164 6935(69) MHz T−1 Atomic & nuclear μn Neutron magnetic moment −9.662 3653(23) × 10−27 J T−1 Atomic & nuclear μn/μB Neutron magnetic moment to Bohr magneton ratio −1.041 875 65(25) × 10−3