Deuteron-proton mass ratio

$m_d/mₚ$ = 1.999 007 501 2699(84)

Value: 1.999 007 501 2699
Standard uncertainty: 8.4 × 10⁻¹²
Relative uncertainty: 4.2 × 10⁻¹² (4.2 parts per trillion)
Status: Measured
Symbol: $m_d/mₚ$
Source: CODATA 2022 via NIST · NIST entry

Use in a calculation

# deuteron-proton mass ratio, CODATA 2022 (NIST)
deuteron_proton_mass_ratio = 1.9990075012699  # standard uncertainty 8.4e-12

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

What it is

The mass of the deuteron 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	1.999 007 501 2699(84)	4.2 × 10⁻¹²	−1.1 σ of 2018
2018	1.999 007 501 39(11)	5.5 × 10⁻¹¹	+2.7 σ of 2014
2014	1.999 007 500 87(19)	9.5 × 10⁻¹¹	−0.6 σ of 2010
2010	1.999 007 500 97(18)	9.0 × 10⁻¹¹	−0.5 σ of 2006
2006	1.999 007 501 08(22)	1.1 × 10⁻¹⁰	+0.6 σ of 2002
2002	1.999 007 500 82(41)	2.1 × 10⁻¹⁰	−0.0 σ of 1998
1998	1.999 007 500 83(41)	2.1 × 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).