Timeline of International Kilogram Prototypes
Physical artifacts that formerly realized and disseminated the kilogram
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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)
| # | 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] |
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