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1kg with creditcard.JPG
Domestic-quality one-kilogram cast iron weight, shaped in accordance with OIML recommendation R52 for cast-iron hexagonal weights, alongside a credit card for scale
Unit system SI unit
Unit of Mass
Symbol kg 
1 kg in ... ... is equal to ...
   Avoirdupois    ≈ 2.205 pounds
   Natural units    ≈ 4.59×107 Planck masses
1.356392608(60)×1050 hertz
External images
BIPM: The IPK in three nested bell jars
NIST: K20, the US National Prototype Kilogram resting on an egg crate fluorescent light panel
BIPM: Steam cleaning a 1 kg prototype before a mass comparison
BIPM: The IPK and its six sister copies in their vault
The Age: Silicon sphere for the Avogadro Project
NPL: The NPL's Watt Balance project
NIST: This particular Rueprecht Balance, an Austrian-made precision balance, was used by the NIST from 1945 until 1960
BIPM: The FB‑2 flexure-strip balance, the BIPM's modern precision balance featuring a standard deviation of one ten-billionth of a kilogram (0.1 µg)
BIPM: Mettler HK1000 balance, featuring 1 µg resolution and a 4 kg maximum mass. Also used by NIST and Sandia National Laboratories' Primary Standards Laboratory
Micro-g LaCoste: FG‑5 absolute gravimeter, (diagram), used in national laboratories to measure gravity to 2 µGal accuracy

The kilogram or kilogramme (SI unit symbol: kg) is the base unit of mass in the International System of Units (SI) (the Metric system) and is defined as being equal to the mass of the International Prototype of the Kilogram (IPK, also known as "Le Grand K" or "Big K").

The avoirdupois (or international) pound, used in both the imperial and US customary systems, is defined as exactly 0.45359237 kg, making one kilogram approximately equal to 2.2046 avoirdupois pounds. Other traditional units of weight and mass around the world are also defined in terms of the kilogram, making the IPK the primary standard for virtually all units of mass on Earth.

The gram, 1/1000 of a kilogram, was provisionally defined in 1795 as the mass of one cubic centimeter of water at the melting point of ice. The final kilogram, manufactured as a prototype in 1799 and from which the IPK was derived in 1875, had a mass equal to the mass of 1 dm3 of water at its maximum density, approximately 4 °C.

The kilogram is the only SI base unit with an SI prefix ("kilo", symbol "k") as part of its name. It is also the only SI unit that is still directly defined by an artifact rather than a fundamental physical property that can be 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 relative to the kilogram, so its stability is important. Only 8 other units do not require the kilogram in their definition: temperature (K, °C), time and frequency (s, Hz, Bq), length (m), and angle (rad, sr).

SI multiples for gram (g)
Submultiples Multiples
Value SI symbol Name Value SI symbol Name
10−1 g dg decigram 101 g dag decagram
10−2 g cg centigram 102 g hg hectogram
10−3 g mg milligram 103 g kg kilogram
10−6 g µg microgram 106 g Mg megagram (tonne)
10−9 g ng nanogram 109 g Gg gigagram
10−12 g pg picogram 1012 g Tg teragram
10−15 g fg femtogram 1015 g Pg petagram
10−18 g ag attogram 1018 g Eg exagram
10−21 g zg zeptogram 1021 g Zg zettagram
10−24 g yg yoctogram 1024 g Yg yottagram
Common prefixed units are in bold face.

In the following sections, wherever numeric equalities are shown in 'concise form'—such as 1.85487(14)×1013—the two digits between the parentheses denote the uncertainty at 1σ standard deviation (68% confidence level) in the two least significant digits of the significand. A final X in a proposed definition denotes digits yet to be agreed on.
  • The microgram is typically abbreviated "mcg" in pharmaceutical and nutritional supplement labelling, to avoid confusion, since the "µ" prefix is not always well recognized outside of technical disciplines. (The expression "mcg" is also the symbol for an obsolete CGS unit of measure known as the "millicentigram", which is equal to 10 µg.)
  • In the UK, because serious medication errors have been made from the confusion between milligrams and micrograms when micrograms has been abbreviated, the recommendation given in the Scottish Palliative Care Guidelines is that doses of less than one milligram must be expressed in micrograms and that the word microgram must be written in full, and that it is never acceptable to use "mcg" or "μg".[2]
  • The decagram (dag in SI) is in much of Europe often abbreviated "dkg" (from the local spelling "dekagram") and is used for typical retail quantities of food (such as cheese and meat).
  • The unit name "megagram" is rarely used, and even then typically only in technical fields in contexts where especially rigorous consistency with the SI standard is desired. For most purposes, the name "tonne" is instead used. The tonne and its symbol, "t", were adopted by the CIPM in 1879. It is a non-SI unit accepted by the BIPM for use with the SI. According to the BIPM, "In English speaking countries this unit is usually called 'metric ton'." The unit name "megatonne" or "megaton" (Mt) is often used in general-interest literature on greenhouse gas emissions, whereas the equivalent unit in scientific papers on the subject is often the "teragram" (Tg).
  • Abstracted: Isolated and its effect changed in form, often simplified or made more accessible in the process.
  • Artifact: A simple human-made object used directly as a comparative standard in the measurement of a physical quantity.
  • Check standard:
    1. A standard body's backup replica of the international prototype kilogram (IPK).
    2. A secondary kilogram mass standard used as a stand-in for the primary standard during routine calibrations.
  • Definition: A formal, specific, and exact specification.
  • Delineation: The physical means used to mark a boundary or express the magnitude of an entity.
  • Disseminate: To widely distribute the magnitude of a unit of measure, typically via replicas and transfer standards.
  • IPK: Abbreviation of "international prototype kilogram", the unique physical object, kept in France, which is internationally recognized as having the defining mass of precisely one kilogram.
  • Magnitude: The extent or numeric value of a property
  • National prototype: A replica of the IPK possessed by a nation.
  • Practical realization: A readily reproducible apparatus to conveniently delineate the magnitude of a unit of measure.
  • Primary national standard:
    1. A replica of the IPK possessed by a nation
    2. The least used replica of the IPK when a nation possesses more than one.
  • Prototype:
    1. A human-made object that serves as the defining comparative standard in the measurement of a physical quantity.
    2. A human-made object that serves as the comparative standard in the measurement of a physical quantity.
    3. The IPK and any of its replicas
  • Replica: An official copy of the IPK.
  • Sister copy: One of six official copies of the IPK that are stored in the same safe as the IPK and are used as check standards by the BIPM.
  • Transfer standard: An artifact or apparatus that reproduces the magnitude of a unit of measure in a different, usually more practical, form.


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