# Definition:Metric System/Mass/Kilogram

(Redirected from Definition:Kilogram)

## Definition

The kilogram is the SI base unit of mass.

It is defined as follows:

$1$ kilogram is the quantity of mass that would make the fixed numerical value of the Planck constant $h$ to be $6 \cdotp 62607015 \times 10^{-34}$ when expressed in the unit Joule seconds.

The Joule second is equal to $1 \, \mathrm {kg} \, \mathrm m^2 \, \mathrm s^{−1}$, where the metre and the second are defined in terms of:

the speed of light $c$
the time of transition between the two hyperfine levels of the ground state of the caesium $133$ atom at rest at $0 \, \mathrm K$.

### Conversion Factors

 $\ds$  $\ds 1$ kilogram $\ds$ $=$ $\ds 1000$ grams $\ds$ $\approx$ $\ds 2 \cdotp 20462$ pounds avoirdupois

### Symbol

$\mathrm {kg}$

The symbol for the kilogram is $\mathrm {kg}$.

The $\LaTeX$ code for $\mathrm {kg}$ is \mathrm {kg} .

## Historical Note

This itself was defined as the mass of one cubic centimetre of water at the melting point of ice.

• 1799: The actual reference kilogram was manufactured as a prototype.

It had a mass equal to the mass of $1 \, \mathrm {dm}^3$ of water at its maximum density, approximately $4 \cels$.

This has a mass which is almost exactly equal to the mass of one litre of water.

The weight of the IPK has been known to vary, and so a more stable alternative was sought, based on a fundamental constant.

• 2011: A decision was reached in principle that it should be redefined in terms of Planck's constant.

The actual decision was deferred to $2014$, and was once then deferred to the next meeting.

## Linguistic Note

The original British English spelling of kilogram was kilogramme.

However, this is rarely used nowadays, as the American kilogram is now the international standard.

## Linguistic Note on Kilo

The prefix kilo- derives from the Greek word χίλιοι (khilioi), which means thousand.