In the CODATA table of physical constants, there are very precise numbers given for mass of electron and mass of proton. And an even more precise number for proton electron mass ratio. But when you divide the mass of proton by the mass of electron, you don't get the same number as the proton electron mass ratio The numbers start diverging at about the 8th significant figure. I'm asking because I need a very precise value of reduced electron mass. So I'm not sure whether to use the standard formula: me * mp / (me + mp) or one with the proton electron mass ratio: mp / (1 + mp/me) They should be the same but they're not and it makes quite a big difference in calculated results.