Timeline of International Kilogram Prototypes

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File:Mass-comparator-with-prototypes.jpg
The BIPM mass comparator with prototype kilograms in place

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]

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)

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# 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]

References

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[15] [37] [11] [1] [2] [9] [7] [16] [17] [29] [12] [13] [18] [24] [28] [31] [32] [22] [10] [20] [27]

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  7. ^ a b F.J. Smith. "Standard Kilogram Weights: A Story of Precision Fabrication". Platinum Metals Rev. 17 (2) (1973) 66‑68.
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  9. ^ a b The International Bureau of Weights and Measures official site: Verifications, retrieved August 4, 2013
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Notes

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  1. ^ The Pavillon's (and hence the BIPM's) postal address is in the neighbouring commune of Sèvres, but the grounds are in Saint-Cloud.
  2. ^ a b Prototype No. 8(41) was accidentally stamped with the number 41, but its accessories carry the proper number 8. Since there is no prototype marked 8, this prototype is referred to as 8(41).
  3. ^ a b Nos. 42′, 77 and 650 are called "standards" rather than "prototypes" because they are slightly underweight, slightly too much material having been removed when they were manufactured. Other than being more than 1 mg below the nominal 1 kg mass, they are identical to the prototypes, and are used during routine calibration work.
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