# Kilogram

Kilogram
General information
Unit systemSI base unit
Unit ofmass
Symbolkg
Conversions
1 kg in ...... is equal to ...
Avoirdupois   ≈ 2.205 pounds[Note 1]
British Gravitational   ≈ 0.0685 slugs

The kilogram (also kilogramme) is the base unit of mass in the metric system, formally the International System of Units (SI), having the unit symbol kg. It is a widely used measure in science, engineering, and commerce worldwide, and is often called a kilo. The kilogram is almost exactly the mass of one litre of water.

The kilogram was originally defined in 1795 as the mass of a litre of water. This was a simple definition, but hard to replicate precisely. In 1799, the Kilogramme des Archives, a platinum artefact, replaced it as the standard of mass. In 1879 a cylinder of platinum-iridium, the International Prototype of the Kilogram (IPK) became the standard of the unit of mass for the metric system, and remained so until May 20, 2019,[1] the last of the metric base units to be defined by a physical artefact.

Despite best efforts to maintain it, evidence accumulated that the mass of the International Prototype Kilogram had been changing; the IPK had diverged from its replicas by approximately 50 micrograms since their manufacture late in the 19th century. This led to several competing efforts to develop measurement technology precise enough to allow replacing the kilogram artefact with a definition based directly on physical fundamental constants.[1] This culminated in 2018 with a definition in terms of the Planck constant.[1] Thus, the kilogram is now defined in terms of the second and the metre, replacing the IPK as primary standard.[2] The new definition was approved by the General Conference on Weights and Measures (CGPM) on November 16, 2018.[3] Physical standard masses such as the IPK and its replicas still serve as secondary standards.

## Definition

The kilogram is defined in terms of three fundamental physical constants: The speed of light c, a specific atomic transition frequency ΔνCs, and the Planck constant h. The formal definition is:

The 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.[2][4]

This definition makes the kilogram consistent with the older definition: the mass remains almost exactly the same as the mass of a litre of water.

### Previous definitions

The development of the first metric system began about 1790. The initial mass unit was the grave, defined in 1793.[5] Within three years it was replaced by 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.[6] The Kilogramme des Archives was manufactured as a prototype in 1799 and served as a basis for the International Prototype Kilogram (IPK) in 1875. It 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.

A replica of the International Prototype Kilogram on display at Cité des Sciences et de l'Industrie, featuring the protective double glass bell. The IPK served as primary standard for the kilogram until 2019.

The International Prototype Kilogram was commissioned by the General Conference on Weights and Measures (CGPM) under the authority of the Metre Convention (1875), and is in the custody of the International Bureau of Weights and Measures (BIPM) who hold it on behalf of the CGPM. The IPK was 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 kilogram was the last SI unit that was directly defined by an artefact rather than fundamental physical properties that could be independently reproduced in different laboratories.[1] 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 were defined in relation to the kilogram, and thus its stability was important. The definitions of only eight other named SI units did not depend on the kilogram: those of temperature (K, °C), time and frequency (s, Hz, Bq), length (m), and angle (rad, sr).[7]

### Replacement of the International Prototype Kilogram

After the International Prototype Kilogram had been found to vary in mass over time relative to its reproductions,[8] 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.[9] CIPM proposed revised definitions of the SI base units for consideration at the 26th CGPM.[10] The formal vote on 16 November 2018 approved the change.[11][12]

Other Languages
Afrikaans: Kilogram
Alemannisch: Kilogramm
Аҧсшәа: Акилограмм
العربية: كيلوغرام
aragonés: Kilogramo
অসমীয়া: কিলোগ্ৰাম
asturianu: Quilogramu
Avañe'ẽ: Kilogarámo
Aymar aru: Kilu
azərbaycanca: Kiloqram
تۆرکجه: کیلوگیرم
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

한국어: 킬로그램
հայերեն: Կիլոգրամ
हिन्दी: किलोग्राम
hrvatski: Kilogram
Bahasa Indonesia: Kilogram
interlingua: Kilogramma
íslenska: Kílógramm
italiano: Chilogrammo
עברית: קילוגרם
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
მარგალური: კილოგრამი
Bahasa Melayu: Kilogram
Mìng-dĕ̤ng-ngṳ̄: Gŭng-gṳ̆ng
монгол: Килограмм
မြန်မာဘာသာ: ကီလိုဂရမ်
Nederlands: Kilogram
नेपाली: किलोग्राम
नेपाल भाषा: किलोग्राम

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
walon: Kilo

Winaray: Kilogramo

ייִדיש: קילאגראם

žemaitėška: Kėluograms