Table 1.

Year of calibration and masses reported by BIPM for the U.S. prototypes

Year	K20	K4
1889	1 kg – 0.039 mg	1 kg – 0.075 mg
1937	1 kg – 0.021 mg
1948	1 kg – 0.019 mg
1984	1 kg – 0.022 mg	1 kg – 0.106 mg
1992	1 kg – 0.021 mg
1999	1 kg – 0.039 mg	1 kg – 0.116 mg