A computer-generated image of the international prototype kilogram (the inch ruler is for scale). The prototype is manufactured from a platinumiridium alloy and is 39.17 mm in both diameter and height, its edges have a four-angle (22.5°, 45°, 67.5° and 79°) chamfer to minimize wear
General information
Unit systemSI base unit
Unit ofmass
1 kg in ...... is equal to ...
   Avoirdupois   ≈ 2.205 pounds[Note 1]
   British Gravitational   ≈ 0.0685 slugs
Official definition
DefinitionThe kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.
2019 kilogram
NIST-4 Kibble balance.jpg
Kibble balance at NIST. Devices like this can weigh objects using the new definition of the kilogram in terms of the Planck constant.
General information
Unit systemSI base unit
Unit ofmass
1 kg in ...... is equal to ...
   Avoirdupois   ≈ 2.205 pounds[Note 2]
   British Gravitational   ≈ 0.0685 slugs
Official definition
Defining authorityInternational Committee for Weights and Measures
Defining event2018 General Conference on Weights and Measures
Effective since20 May 2019
DefinitionThe kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015×10−34 when expressed in the unit J⋅s, which is equal to kg⋅m2⋅s−1, where the metre and the second are defined in terms of c and ΔνCs.
Domestic-quality one-kilogram cast iron weight, shaped in accordance with OIML recommendation R52 for cast-iron hexagonal weights,[1] alongside a credit card for size comparison

The kilogram or kilogramme (symbol: kg) is the base unit of mass in the International System of Units (SI). Until 20 May 2019, it remains defined by a platinum alloy cylinder, the International Prototype Kilogram (informally Le Grand K or IPK), manufactured in 1889, and carefully stored in Saint-Cloud, a suburb of Paris. After 20 May, it will be defined in terms of fundamental physical constants.

The kilogram was originally defined as the mass of a litre (cubic decimetre) of water. That was an inconvenient quantity to precisely replicate, so in 1799 a platinum artefact was fashioned to define the kilogram. That artefact, and the later IPK, have been the standard of the unit of mass for the metric system ever since.

In spite of best efforts to maintain it, the IPK has diverged from its replicas by approximately 50 micrograms since their manufacture late in the 19th century. This led to efforts to develop measurement technology precise enough to allow replacing the kilogram artifact with a definition based directly on physical phenomena, a process which is scheduled to finally take place in 2019.

The new definition is based on invariant constants of nature, in particular the Planck constant which will change to being defined rather than measured, thereby fixing the value of the kilogram in terms of the second and the metre, and eliminating the need for the IPK.[2] The new definition was approved by the General Conference on Weights and Measures (CGPM) on 16 November 2018.[3] The Planck constant relates a light particle’s energy, and hence mass, to its frequency. The new definition only became possible when instruments were devised to measure the Planck constant with sufficient accuracy based on the IPK definition of the kilogram.


The gram, 1/1000 of a kilogram, was provisionally defined in 1795 as the mass of one cubic centimetre of water at the melting point of ice.[4] The final kilogram, manufactured as a prototype in 1799 and from which the International Prototype Kilogram (IPK) was derived in 1875, had a mass equal to the mass of 1 dm3 of water under atmospheric pressure and at the temperature of its maximum density, which is approximately 4 °C.

The kilogram is the only named SI unit with an SI prefix (kilo) as part of its name. Until the 2019 redefinition of SI base units, it was also the last SI unit that was still directly defined by an artefact rather than a fundamental physical property that could be independently reproduced in different laboratories. Three other base units (cd, A, mol) and 17 derived units (N, Pa, J, W, C, V, F, Ω, S, Wb, T, H, kat, Gy, Sv, lm, lx) in the SI system are defined in relation to the kilogram, and thus its stability is important. The definitions of only eight other named SI units do not depend on the kilogram: those of temperature (K, °C), time and frequency (s, Hz, Bq), length (m), and angle (rad, sr).[5]

The IPK is rarely used or handled. Copies of the IPK kept by national metrology laboratories around the world were compared with the IPK in 1889, 1948, and 1989 to provide traceability of measurements of mass anywhere in the world back to the IPK.

The International Prototype Kilogram was commissioned by the General Conference on Weights and Measures (CGPM) under the authority of the Metre Convention (1875), and in the custody of the International Bureau of Weights and Measures (BIPM) who hold it on behalf of the CGPM. After the International Prototype Kilogram had been found to vary in mass over time relative to its reproductions, the International Committee for Weights and Measures (CIPM) recommended in 2005 that the kilogram be redefined in terms of a fundamental constant of nature. At its 2011 meeting, the CGPM agreed in principle that the kilogram should be redefined in terms of the Planck constant, h. The decision was originally deferred until 2014; in 2014 it was deferred again until the next meeting.[6] CIPM has proposed[7] revised definitions of the SI base units, for consideration at the 26th CGPM. The formal vote, which took place on 16 November 2018,[8] approved the change,[9] with the new definitions coming into force on 20 May 2019.[10] The accepted redefinition defines the Planck Constant as exactly 6.62607015×10−34 kg⋅m2⋅s−1, thereby defining the kilogram in terms of the second and the metre.[2] Since the metre is defined as a time fraction of the speed of light in vacuum, then the kilogram is defined in terms of the time only.

The avoirdupois (or international) pound, used in both the imperial and US customary systems, is now defined in terms of the kilogram.[11] Other traditional units of weight and mass around the world are now also defined in terms of the kilogram, making the kilogram the primary standard for virtually all units of mass on Earth.

Other Languages
Afrikaans: Kilogram
Alemannisch: Kilogramm
Аҧсшәа: Акилограмм
العربية: كيلوغرام
aragonés: Kilogramo
অসমীয়া: কিলোগ্ৰাম
asturianu: Quilogramu
Avañe'ẽ: Kilogarámo
Aymar aru: Kilu
azərbaycanca: Kiloqram
Bân-lâm-gú: Kong-kin
башҡортса: Килограмм
беларуская: Кілаграм
беларуская (тарашкевіца)‎: Кіляграм
भोजपुरी: किलोग्राम
български: Килограм
བོད་ཡིག: སྟོང་ཁེའུ།
bosanski: Kilogram
brezhoneg: Kilogramm
català: Quilogram
Чӑвашла: Килограмм
čeština: Kilogram
Cymraeg: Cilogram
dansk: Kilogram
Deutsch: Kilogramm
eesti: Kilogramm
Ελληνικά: Χιλιόγραμμο
español: Kilogramo
Esperanto: Kilogramo
euskara: Kilogramo
فارسی: کیلوگرم
français: Kilogramme
Frysk: Kilogram
furlan: Chilogram
Gaeilge: Cileagram
galego: Quilogramo
贛語: 公斤
客家語/Hak-kâ-ngî: Kûng-kîn
한국어: 킬로그램
հայերեն: Կիլոգրամ
हिन्दी: किलोग्राम
hrvatski: Kilogram
Bahasa Indonesia: Kilogram
interlingua: Kilogramma
íslenska: Kílógramm
italiano: Chilogrammo
עברית: קילוגרם
Basa Jawa: Kilogram
къарачай-малкъар: Килограмм
ქართული: კილოგრამი
қазақша: Килограмм
Kiswahili: Kilogramu
Kreyòl ayisyen: Kilogram
kurdî: Kîlogram
Кыргызча: Килограмм
Latina: Chiliogramma
latviešu: Kilograms
Lëtzebuergesch: Kilogramm
lietuvių: Kilogramas
Limburgs: Kilogram
lingála: Kilogálame
lumbaart: Chilogram
magyar: Kilogramm
македонски: Килограм
മലയാളം: കിലോഗ്രാം
Malti: Kilogramm
Bahasa Melayu: Kilogram
Mìng-dĕ̤ng-ngṳ̄: Gŭng-gṳ̆ng
монгол: Килограмм
မြန်မာဘာသာ: ကီလိုဂရမ်
Nederlands: Kilogram
नेपाली: किलोग्राम
नेपाल भाषा: किलोग्राम
日本語: キログラム
Nordfriisk: Kilogram
norsk: Kilogram
norsk nynorsk: Kilogram
occitan: Quilograma
oʻzbekcha/ўзбекча: Kilogramm
ਪੰਜਾਬੀ: ਕਿਲੋਗ੍ਰਾਮ
پنجابی: کلوگرام
Patois: Kilogram
Plattdüütsch: Kilogramm
polski: Kilogram
português: Quilograma
română: Kilogram
Runa Simi: Kilugramu
русиньскый: Килограм
русский: Килограмм
Gagana Samoa: Kilokalama
संस्कृतम्: किलोग्राम्
Scots: Kilogramme
shqip: Kilogrami
sicilianu: Chilugrammu
Simple English: Kilogram
slovenčina: Kilogram
slovenščina: Kilogram
ślůnski: Kilogram
Soomaaliga: Kiilogaraam
کوردی: کیلۆگرام
српски / srpski: Килограм
srpskohrvatski / српскохрватски: Kilogram
Basa Sunda: Kilogram
suomi: Kilogramma
svenska: Kilogram
Tagalog: Kilogramo
తెలుగు: కిలోగ్రాము
Türkçe: Kilogram
українська: Кілограм
اردو: کلوگرام
vèneto: Chiłogramo
Tiếng Việt: Kilôgam
文言: 公斤
Winaray: Kilogramo
吴语: 公斤
ייִדיש: קילאגראם
粵語: 千克
žemaitėška: Kėluograms
中文: 千克