# Which is the right electron mass?

When particles mass can be changed by changing the website, then how to calculate with confidence?

For example: Google: electron mass = 9.10938 188 × 10$^-31$ kilograms

Wikipedia: electron mass

9.109 38 2 15(45) ×10$^−31$ kg

5.485 799 0943(23)×10$^−4$ u

8.187 104 38(41)×10$^−14$ J/c$^2$

0.510 998 910(13) MeV/c$^2$

The difference is small, but it makes little difference in accuracy.

• wikipedia in the electron entry 9.10938291(40)×10−31 kg 9.10938291(40)×10−31 kg . They are all the same within errors, and the errors come from various experiments measuring the mass with an error. no problem, that is why errors are important to be quoted Feb 11, 2013 at 18:03
• From NIST (9.1093837015E-31)×(2.99792458E8**2) ÷(1.602176634E-19)÷1E6 =0.510998949996164 MeV Mar 3, 2022 at 1:15
• Google Calculator is very handy, but please be cautious of the values it uses for physical constants. Its current value for the electron mass is $9.10938356 × 10^{-31}$ kilograms. Mar 3, 2022 at 2:44
• For some unknown reason, its definition of the lightyear uses the tropical year instead of the Julian year, resulting in the value of ~9.4605284E15 metres, instead of the correct value of 9460730472580800 metres. Mar 3, 2022 at 2:49

Obviously you go to the newest and most authoritative source you can get. As far as I am concerned that is the latest edition of the Particle Data Book which says the correct figure is $0.510998928 \pm 0.000000011\text{ MeV}$.
The NIST web site gives the electron mass as 9.10938291 $\pm$ 0.00000040 $\times$ 10$^{-31}$ kg. The 0.00000040 is a single standard deviation, so Google's value, 9.10938188 $\times$ 10$^{-31}$ kg is (just) within three standard deviations. The figures aren't that different.