Timeline of International Kilogram Prototypes

Official copies of the International Prototype of the Kilogram (IPK), the 1 kg platinum–iridium alloy right circular cylinders, disseminated the kilogram from 1889^[1] until the redefinition based on physical constants in 2019.^[2] These prototypes underpinned global trust in scientific discovery, industrial manufacturing, and international trade for over a century.^[3][4]

The BIPM mass comparator with prototype kilograms in place

Under the Metre Convention’s framework for international collaboration in metrology, the pure platinum "Kilogram of the Archives" standard from 1799 was replaced by the platinum–iridium International Prototype of the Kilogram (IPK) in 1879. Pure platinum was too soft for a durable mass standard, but the addition of just 10% iridium in the alloy greatly increased hardness while still retaining extreme resistance to oxidation, extremely high density, and low magnetic susceptibility.^[5] The harder alloy reduced wear and allowed the prototypes to be finished to a high polish, minimising variability.^[6]

The IPK and six sister copies are stored under secure environmental controls at the International Bureau of Weights and Measures (BIPM) in the Pavillon de Breteuil.^{[Note 1]} Other copies, manufactured primarily by Johnson Matthey beginning in 1879,^[7] were distributed to national metrology institutes of countries that had ratified and conformed to the Treaty of the Metre (and to certain non‑national organisations).^[8] Each copy carries a unique identification number and served as a primary mass standard, providing traceability of local measurements to the IPK through periodic comparisons.^[9]

The timeline shows the year of assignment and the year of last known calibration. The entries fall into three broad groups:

• Copies 0–40 — Foundational prototypes and early national standards: the IPK itself, its six BIPM sister copies, and the first wave of official allocations to original signatories after the 1st CGPM, with detailed custody and calibration histories.
• Copies 44–63 — Mid‑period issues and expanding membership: mid‑career Johnson Matthey productions allocated to new member states, as well as replacement or supplementary prototypes.
• Copies 75–special designations — Late‑period and special‑purpose prototypes: later allocations, non‑sequential or experimental artefacts, and prototypes intended for particular scientific or commemorative purposes, each with its own custodial context.

List of official copies of the International Prototype of the Kilogram (IPK)

#	Allocation: Country (year)	Last known location (institution, city)	Last known status	Last known Calibration	Drift (µg/yr)	Notes
𝕶	BIPM (1883)[6]	BIPM vault, Saint-Cloud, France	International Prototype Kilogram (IPK) "Le Grand K (𝕶)"[6]	2014[10]	−0.50	Drift ≈ −0.50 μg/yr over ~100 yr relative to official copies; ~+50 μg total divergence; stored under triple bell jars.[11][12][13]
KI	BIPM (1889)[6]	BIPM vault, Saint‑Cloud, France	Check standard	2014[10]		First of three nearly identical cylinders of Pt10Ir alloy fabricated in 1879.[6]
KII		Paris Observatory, Paris[6]		1880[6]		Compared to Kilogram of the Archives w/KI & KIII (later 𝕶) by four observers in 1880 at the Paris Observatory.[6]
1	France (1889)	BIPM vault, Saint‑Cloud, France	Retired check standard1889-1925 (“sister” copy).[14]	1988-92[14]		Retired in 1925 after falling. Replaced as an official copy by No. 7.[14]
2	Romania (1889)	National repository, Romania	National prototype	2023 No. 165[15]
3	Spain (1889)[8]	National repository, Spain	National prototype	2013 No. 48[15]
4	United States (1889)[8]	NIST, United States	National prototype[16]	2013 No. 47[17]	−0.37	1889: -75 μg; 1989: -106 μg; ~1999: -116 μg. Net change −41 μg over ~110 yr.[18]
5	Italy (1889)[8]	National repository, Italy	National prototype	1988-92[14]
6	Japan (1889)[8]	National Metrology Institute, Japan	National prototype	1988-92[14]
7	France	BIPM vault, Saint‑Cloud, France	Official copy since 1925[10] (“sister” copy)	1988-92[14]
8	France	BIPM vault, Saint‑Cloud, France	Official copy since 1905[10] (serial 8(41))	1988-92[14]		Sister copy of IPK #8 mis‑stamped as 41[Note 2]
9	BIPM (1889)[8]	BIPM calibration laboratory, Saint‑Cloud, France	Working copy 2014[10]	2014[10]		Fell in Bunge balance 1949.[14] Used for BIPM 1963–64 verification.[19]
10	Portugal (1889)[8]	IPQ, Caparica	National prototype	2022 No. 110[15]
11	Serbia (1889)[8]	National repository, Serbia	National prototype retired.			Damaged 1907; 1925 replaced by No. 29.[14]
12	Russia (1889)[8]	VNIIM, Saint Petersburg, Russia	National prototype[20]	2014 No. 24[15]
13	France (1889)[8]
14	Austria (1889)[8]	BEV	National prototype	2019 No. 46[15]
15	Bavaria (1889)[8]			1954[21]
16	Hungary (1889)[8]	National repository, Hungary	National prototype	2018 No. 39[15]
17	France (1889)[8]	LNE-CETIAT	National prototype	2015 No. 18[15]
18	United Kingdom (1889)[8]	National Physical Laboratory, United Kingdom	National prototype[22]	2024 No. 11
19	Italy (1889)[8]			1954[21]
20	United States (1889)[8]	NIST, United States	National prototype[16]	2019 No. 44[15]	0.00	1889: -39 μg; 1948: -19 μg; 1989: -39 μg (no net drift over ~100 yr).[18]
21	Mexico	National repository, Mexico	National prototype	2021 No. 107[15]
22	Germany (1889)[8]			1903[23]		Damaged in Berlin in 1944[14]
23	Finland	National repository, Finland	National prototype	2022 No. 108[15]
24	Spain (1889)[8]	National repository, Spain	National prototype	2015 No. 121[15]
25	Paris Observatory (1889)[8]	BIPM calibration laboratory, Saint‑Cloud	Working copy (special use)[19]	2014[10]		Re‑assigned 1958, to the BIPM in nearly unused condition.[19]
26	Saint Petersburg Academy of Sci. (1889)[8]	VNIIM, Saint Petersburg, Russia	Official copy[20]
27	Denmark (1889)[8]					Retired 1946; 1949 replaced by No. 48,[14]
28	Belgium (1889)[8]	National repository, Belgium	National prototype	2021 No. 16[15]
29	Conservatoire national des arts et métiers	National repository, Serbia	National prototype			1925 transferred from Conservatoire national des arts et métiers to Kingdom of Yugoslavia to replace No. 11.[14]
30	Argentina	National repository, Argentina	National prototype	2019 No. 6[15]
31	BIPM (1889)[8]	BIPM calibration laboratory, Saint‑Cloud, France	Working copy	2014[10]		Fell in Bunge balance 1951.[14] Used for BIPM 1963–64 verification[19]
32	France	BIPM vault, Saint‑Cloud, France	Official copy since 1905[10] (“sister” copy)	2014[10]	0.00	BIPM cleaning trials (2014) on Nos. 7 and 32 showed < 0.5 μg mass loss on a third full cleaning; high short‑term stability.[10][24]
33	Austria (1889)[8]
34	France (1889)[8]	French Academy of Sciences, Paris, France[15]	Held by non‑national organisation[17]	2015 No. 67[15]
35	France (1889)[8]	National repository, France	National prototype	2015 No. 74[15]
36	Norway (1889)[8]	National repository, Norway	National prototype	2024 No. 104[15]
37	Belgium (1889)[8]	National repository, Belgium	National prototype	2022 No. 112[15]
38	Switzerland (1889)[8]	National repository, Switzerland	National prototype	2015 No. 20[15]
39	Japan	National repository, South Korea	National prototype	1988-92[14]	−6.47	−665 μg by the 3rd PV with poor surface condition.[14] Ceded to the Republic of Korea in 1958.[11]
40	Sweden (1889)[8]	RISE, Borås	National prototype	1991 No. 22[25]	-0.02	1889: -0.037; 1948: -0.039, 1991: -0.035(2) mg[25]
41	Czechoslovakia[26](1929)[14]	Slovak Republic		1954		Shown in 1st PV unassigned,[23] 2nd PV assigned to CS, but not in 3rd.[14] No 8(41) appears separately.[26] [Note 2]
42′	France	BIPM, Saint‑Cloud, France	Underweight “standard” (working copy)[Note 3][10]	2014[10]		Allocated to Turkey in 1935; 1953 exchanged for No 54.[14]
43	France	BIPM, Saint‑Cloud, France	Official copy since 1939[10] (“sister” copy)[27]	1988-92[14]		Used for BIPM 1963–64 verification[19]
44	Australia (1947)[14]	NMI, Australia	National prototype[11][17]	2015 No. 70[15]		Returned for BIPM 1963–64 verification on request[19]
45	Argentina[19] (1939)[14]					Not returned for BIPM 1963–64 verification on request[19] Lost 1986.[14]
46	Dutch East Indies (1939)[14]	SNSU‑BSN, Indonesia	National prototype[11][17]	2015 No. 82[15]		Returned for BIPM 1963–64 verification on request[19]
47	France	BIPM, Saint‑Cloud, France	Official copy since 1939[10] (“sister” copy)[27]	2014[10]		Used for BIPM 1963–64 verification[19]
48	Denmark (1949)[14]	DFM, Denmark	National prototype[11][17]	2021 No. 106[15]	+0.78	Delivered in 1949 at +81 μg; certified +112 μg in the 1988–1992 periodic verification (+31 μg over ~40 yr).[11][28][12]
49	Austria	BEV, Austria	National prototype[11][17]	2019 No. 46[15]		Returned for BIPM 1963–64 verification on request[19]
50	Canada	NRC, Canada	National prototype[11][17]	1988-92[14]		Returned for BIPM 1963–64 verification on request[19]
51	Poland	GUM, Poland	National prototype[11][17]	2021 No. 4[15]
52	Germany	PTB, Germany	National prototype[11][17]	2010 No. 21[17]
53	Netherlands	VSL, Netherlands	National prototype[11][17]	2024 No. 12		1889: Kingdom of the Netherlands had ratified but didn't adhere to the Meter Treaty and was not entitled to an original prototype.[8]
54	Turkey (1953)[14]	UME, Turkey	National prototype[11][17][29]	2017 No. 48[15]		Exchanged to replace No 42.[14]
55	Germany (1954)[30]	PTB, Germany	National prototype[11][17]	2023 No. 80[15]	+1.26	~126 μg/century relative to the IPK since 1954; cited as a high‑drift case.[31][32][12]
56	South Africa (1955)	NMISA, South Africa	National prototype[11][17]	2015 No. 77[15]
57	India (1958)[33]	NPLI, New Delhi	National prototype[11][17]	2022 No. 109[15]		Excluded from 3rd PV cleaning mass change calculations.[14]
58	United Arab Republic[19] (1963)[14]	NIS, Egypt	National prototype[11][17]	2022 No. 115[15]		Used for BIPM 1963–64 verification[19]
59	n/a	National Research Laboratory of Metrology (Japan)	Standard E59			Mistakenly listed as platinum-iridium prototype; NUM number E59.[14]
60	China (1963)[14]	NIM, China	National prototype[11][17]	2023 No. 164[15]
61	China (1963)[14]					Retired 1978; replaced by No. 64.[14]
62	Istituto di Metrologia G. Colonnetti (1964)[14]	INRiM, Turin	Held by non‑national organisation[11][17]	1988-92[14]
63	France	BIPM, Saint‑Cloud, France	Working copy[10]	2014[10]	0.00	~2 μg stability across repeated cleanings using the NBS‑2 balance.[10][24]
64	China	NIM, China	National prototype[11][17]	2024 No. 103
65	Slovakia	SMU, Slovakia	National prototype[11][17]	2019 No. 47[15]
66	Brazil	INMETRO, Brazil	National prototype[11][17]	2019 No. 3[15]
67	Czech Republic	CMI, Czech Republic	National prototype[11][17]	2021 No. 2[15]
68	Chile	CENAMET, Chile	National prototype[11][17]	1988-92[14]
69	Portugal	Portuguese Institute for Quality, Portugal	National prototype[11][17]	2022 No. 111[15]
70	Germany	PTB, Germany	National prototype[11][17]	2013 No. 70[17]
71	Israel (1992)[14]	INMI, Israel	National prototype[11][17]	2015 No. 83[15]		Newly machined, finished and adjusted @BIPM w/diamond tool.[14]
72	South Korea (1992)[14]	KRISS, South Korea	National prototype[11][17]	2017 No. 45[15]		Newly machined, finished and adjusted @BIPM w/diamond tool.[14]
73	BIPM (1988)[14]	BIPM, Saint‑Cloud, France	Special‑use working copy[10]	2014[10]
74	Canada (1992)[14]	NRC, Canada	National prototype[11][17]	2015 No. 23[15]		Newly machined, finished and adjusted @BIPM w/diamond tool.[14]
75	Hong Kong (1993)[34]	SCL, Hong Kong[17]	National prototype	2017 No. 1[15]		Newly machined, finished and adjusted @BIPM w/diamond tool.[14]
76	Istituto di metrologia G. Colonnetti (1993)	INRiM, Turin	National prototype[11][17]	2025 No. 126
77	France (1992)[14]	BIPM, Saint‑Cloud, France	Underweight “standard”[Note 3][10]	2014[10]		Underweight standard; not included in drift calculations.[11][28]
78	Taiwan	Center for Measurement Standards (CMS), ITRI, Hsinchu[35]	National prototype[11][17]	2025 No. 40
79	United States (1996)[36]	NIST, United States	National prototype[11][17]	2012 No. 4[17]
80	Thailand	NIMT, Thailand	National prototype[11][17]	2023 No. 84[15]
81	United Kingdom	NPL, United Kingdom	National prototype[11][17]	2013 No. 47[17]
82	United Kingdom	NPL, United Kingdom	National prototype[11][17]	2004 No. 20[17]
83	Singapore	NMC, Singapore	National prototype[11][17]	2023 No. 75[15]
84	South Korea	KRISS, South Korea	National prototype[11][17]	2003 No. 53[17]
85	United States	NIST,[37] United States	National prototype[11][17]	2012 No. 30[17]		Used in 2019-20 CCM key comparison
86	Sweden (2002)[25]	RISE, Borås	National prototype[11][17]	2004 No. 68[25]
87	Australia	NMI, Australia	National prototype[11][17]	2004 No. 77[17]
88	France	BIPM, Saint‑Cloud, France	Working copy[10]	2014[10]
89	Switzerland	METAS, Switzerland	National prototype[11][17]	2015 No. 73[15]
90	Mexico	CENAM, Mexico	National prototype[11][17]	2021 No. 108[15]
91	France	BIPM, Saint‑Cloud, France	Working copy[10]	2014[10]
92	United States	NIST, United States	National prototype[11][17]	2016 No. 134[15]
93	Saudi Arabia	SASO, Riyadh	National prototype[11][17]	2024 No. 13[15]
94	Japan	NMIJ,[37] Tsukuba, Japan	National prototype[11][17]	2015 No. 25[15]		Used in 2019-20 CCM key comparison
95	Kenya	KEBS, Kenya	National prototype[11][17]	2022 No. 133[15]
96	Mexico	CENAM, Mexico	National prototype[11][17]	2021 No. 108[15]
97
98
99
100
101
102	USA	NIST, USA[17]		2014 No. 53[15]
103
104	USA	NIST, USA[37]		2014 No. 54[15]		Used in 2019-20 CCM key comparison
105	USA	NIST, USA[17]		2014 No. 55[15]
106	Canada	NRC, Canada[37]		2015 No. 17[15]		Used in 2019-20 CCM key comparison
107	Pakistan[15]			2017 No. 47[15]
108	Colombia[15]			2015 No. 127[15]
109	Germany	PTB, Germany[37]		2015 No. 5[15]		Used in 2019-20 CCM key comparison
110	China	NIM, China[37]		2016 No. 74[15]		Used in 2019-20 CCM key comparison
111	South Korea	KRISS, South Korea[37]		2017 No. 46[15]		Used in 2019-20 CCM key comparison
112	Indonesia	SNSU‑BSN, Indonesia		2019 No. 98[15]
113
114	China (2022)[38]	NIM, China		2022 No. 23[15]
115	China (2022)	NIM, China		2022 No. 36[15]
650	BIPM		Working copy since 1993[10]	2014[10]