The NIST Reference on Constants, Units and Uncertainty

Fundamental Physical Constants
Constants
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Correlation coefficient between two constants
proton mass
 $m_{\rm p}$
 Numerical value  1.672 621 923 69 x 10-27 kg
 Standard uncertainty  0.000 000 000 51 x 10-27 kg
  Relative standard uncertainty  3.1 x 10-10
 Concise form  1.672 621 923 69(51) x 10-27 kg    
 
Source: 2018 CODATA     
recommended values 		 Definition of
uncertainty 		       Correlation coefficient with
any other constant

 
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