Official copies of the International Prototype of the Kilogram (IPK), the 1 kg platinum–iridium alloy right circular cylinders, disseminated the kilogram from 1889 until the redefinition based on physical constants in 2019. These prototypes underpinned global trust in scientific discovery, industrial manufacturing, and international trade for over a century.
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. The harder alloy reduced wear and allowed the prototypes to be finished to a high polish, minimising variability.
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.The Pavillon's (and hence the BIPM's) postal address is in the neighbouring commune of Sèvres, but the grounds are in Saint-Cloud. Other copies, manufactured primarily by Johnson Matthey beginning in 1879, 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). 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.
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 knownstatusLast knownCalibration Drift(μg/yr) Notes 𝕶 BIPM (1883) BIPM vault, Saint-Cloud, France International Prototype Kilogram (IPK) "Le Grand K (𝕶)" 2014 −0.50 Drift ≈ −0.50 μg/yr over ~100 yr relative to official copies; ~+50 μg total divergence; stored under triple bell jars.KIBIPM (1889)BIPM vault, Saint‑Cloud, FranceCheck standard 2014First of three nearly identical cylinders of Pt10Ir alloy fabricated in 1879.KIIParis Observatory, Paris1880Compared to Kilogram of the Archives w/KI & KIII (later 𝕶) by four observers in 1880 at the Paris Observatory. 1 France (1889) BIPM vault, Saint‑Cloud, France Retired check standard1889-1925 ("sister" copy). 1988-92 Retired in 1925 after falling. Replaced as an official copy by No. 7. 2 Romania (1889) National repository, Romania National prototype 2023 No. 165 3 Spain (1889) National repository, Spain National prototype 2013 No. 48 4 United States (1889) NIST, United States National prototype 2013 No. 47 −0.37 1889: -75 μg; 1989: -106 μg; ~1999: -116 μg. Net change −41 μg over ~110 yr. 5 Italy (1889) National repository, Italy National prototype 1988-92 6 Japan (1889) National Metrology Institute, Japan National prototype 1988-92 7 France BIPM vault, Saint‑Cloud, France Official copy since 1925("sister" copy) 1988-92 8 France BIPM vault, Saint‑Cloud, France Official copy since 1905(serial 8(41)) 1988-92 Sister copy of IPK #8 mis‑stamped as 41Prototype 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). 9 BIPM (1889) BIPM calibration laboratory, Saint‑Cloud, France Working copy 2014 2014 Fell in Bunge balance 1949. Used for BIPM 1963–64 verification. 10 Portugal (1889) IPQ, Caparica National prototype 2022 No. 110 11 Serbia (1889) National repository, Serbia National prototype retired. Damaged 1907; 1925 replaced by No. 29. 12 Russia (1889) VNIIM, Saint Petersburg, Russia National prototype 2014 No. 24 13 France (1889) 14 Austria (1889) BEV National prototype 2019 No. 46 15 Bavaria (1889) 1954 16 Hungary (1889) National repository, Hungary National prototype 2018 No. 39 17 France (1889) LNE-CETIAT National prototype 2015 No. 18 18 United Kingdom (1889) National Physical Laboratory, United Kingdom National prototype 2024 No. 11 19 Italy (1889) 1954 20 United States (1889) NIST, United States National prototype 2019 No. 44 0.00 1889: -39 μg; 1948: -19 μg; 1989: -39 μg (no net drift over ~100 yr). 21 Mexico National repository, Mexico National prototype 2021 No. 107 22 Germany (1889) 1903 Damaged in Berlin in 1944 23 Finland National repository, Finland National prototype 2022 No. 108 24 Spain (1889) National repository, Spain National prototype 2015 No. 121 25 Paris Observatory (1889) BIPM calibration laboratory, Saint‑Cloud Working copy(special use) 2014Re‑assigned 1958, to the BIPM in nearly unused condition. 26 Saint Petersburg Academy of Sci. (1889) VNIIM, Saint Petersburg, Russia Official copy 27 Denmark (1889) Retired 1946; 1949 replaced by No. 48, 28 Belgium (1889) National repository, Belgium National prototype 2021 No. 16 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. 30 Argentina National repository, Argentina National prototype 2019 No. 6 31 BIPM (1889) BIPM calibration laboratory, Saint‑Cloud, France Working copy 2014 Fell in Bunge balance 1951. Used for BIPM 1963–64 verification 32 France BIPM vault, Saint‑Cloud, France Official copy since 1905("sister" copy) 2014 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. 33 Austria (1889) 34 France (1889) French Academy of Sciences, Paris, France Held by non‑national organisation 2015 No. 67 35 France (1889) National repository, France National prototype 2015 No. 74 36 Norway (1889) National repository, Norway National prototype 2024 No. 104 37 Belgium (1889) National repository, Belgium National prototype 2022 No. 112 38 Switzerland (1889) National repository, Switzerland National prototype 2015 No. 20 39 Japan National repository, South Korea National prototype 1988-92 −6.47 −665 μg by the 3rd PV with poor surface condition. Ceded to the Republic of Korea in 1958. 40 Sweden (1889) RISE, Borås National prototype 1991 No. 22 -0.021889: -0.037; 1948: -0.039, 1991: -0.035(2) mg 41 Czechoslovakia(1929) Slovak Republic 1954Shown in 1st PV unassigned, 2nd PV assigned to CS, but not in 3rd. No 8(41) appears separately. 42 France BIPM, Saint‑Cloud, France Underweight "standard" (working copy)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. 2014 Allocated to Turkey in 1935; 1953 exchanged for No 54. 43 France BIPM, Saint‑Cloud, France Official copy since 1939("sister" copy) 1988-92 Used for BIPM 1963–64 verification 44 Australia (1947) NMI, Australia National prototype 2015 No. 70 Returned for BIPM 1963–64 verification on request45Argentina (1939)Not returned for BIPM 1963–64 verification on request Lost 1986. 46 Dutch East Indies (1939) SNSU‑BSN, Indonesia National prototype 2015 No. 82 Returned for BIPM 1963–64 verification on request 47 France BIPM, Saint‑Cloud, France Official copy since 1939("sister" copy) 2014 Used for BIPM 1963–64 verification 48 Denmark (1949) DFM, Denmark National prototype 2021 No. 106 +0.78 Delivered in 1949 at +81 μg; certified +112 μg in the 1988–1992 periodic verification (+31 μg over ~40 yr). 49 Austria BEV, Austria National prototype 2019 No. 46 Returned for BIPM 1963–64 verification on request 50 Canada NRC, Canada National prototype 1988-92 Returned for BIPM 1963–64 verification on request 51 Poland GUM, Poland National prototype 2021 No. 4 52 Germany PTB, Germany National prototype 2010 No. 21 53 Netherlands VSL, Netherlands National prototype 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. 54 Turkey (1953) UME, Turkey National prototype 2017 No. 48 Exchanged to replace No 42. 55 Germany (1954) PTB, Germany National prototype 2023 No. 80 +1.26 ~126 μg/century relative to the IPK since 1954; cited as a high‑drift case. 56 South Africa (1955) NMISA, South Africa National prototype 2015 No. 77 57 India (1958) quote=India too based its measurement of the Kilogram on the IPK, an official NPK of which came to India in 1958 and is preserved at the CSIR-National Physical Laboratory, New Delhi. NPLI, New Delhi National prototype 2022 No. 109 Excluded from 3rd PV cleaning mass change calculations. 58 United Arab Republic (1963) NIS, Egypt National prototype 2022 No. 115 Used for BIPM 1963–64 verification59n/aNational Research Laboratory of Metrology (Japan)Standard E59Mistakenly listed as platinum-iridium prototype; NUM number E59. 60 China (1963) NIM, China National prototype 2023 No. 164 61China (1963)Retired 1978; replaced by No. 64. 62 Istituto di Metrologia G. Colonnetti (1964) INRiM, Turin Held by non‑national organisation 1988-92 63 France BIPM, Saint‑Cloud, France Working copy 2014 0.00 ~2 μg stability across repeated cleanings using the NBS‑2 balance. 64 China NIM, China National prototype 2024 No. 103 65 Slovakia SMU, Slovakia National prototype 2019 No. 47 66 Brazil INMETRO, Brazil National prototype 2019 No. 3 67 Czech Republic CMI, Czech Republic National prototype 2021 No. 2 68 Chile CENAMET, Chile National prototype 1988-92 69 Portugal Portuguese Institute for Quality, Portugal National prototype 2022 No. 111 70 Germany PTB, Germany National prototype 2013 No. 70 71 Israel (1992) INMI, Israel National prototype 2015 No. 83 Newly machined, finished and adjusted @BIPM w/diamond tool. 72 South Korea (1992) KRISS, South Korea National prototype 2017 No. 45 Newly machined, finished and adjusted @BIPM w/diamond tool. 73 BIPM (1988) BIPM, Saint‑Cloud, France Special‑use working copy 2014 74 Canada (1992) NRC, Canada National prototype 2015 No. 23 Newly machined, finished and adjusted @BIPM w/diamond tool. 75 Hong Kong (1993) SCL, Hong Kong National prototype 2017 No. 1 Newly machined, finished and adjusted @BIPM w/diamond tool. 76 Istituto di metrologia G. Colonnetti (1993) INRiM, Turin National prototype 2025 No. 126 77 France (1992) BIPM, Saint‑Cloud, France Underweight "standard" 2014 Underweight standard; not included in drift calculations. 78 Taiwan Center for Measurement Standards (CMS), ITRI, Hsinchu National prototype 2025 No. 40 79 United States