| Reference | Grams (g) | Kilograms (kg) |
|---|---|---|
| A sheet of A4 paper (80 gsm) | 5 g | 0.005 kg |
| One litre of water | 1000 g | 1 kg |
| Average adult human | 70000 g | 70 kg |
| A small car | 1400000 g | 1400 kg |
The gram is a unit of mass equal to one thousandth of a kilogram. Its symbol is g. Although the kilogram is the SI base unit, the gram is the one that carries the prefixes, so milligram and microgram are built on the gram rather than on the kilogram.
That awkwardness is historical. The original metric system of 1795 defined the gramme as the mass of one cubic centimetre of water at the temperature of melting ice. It was intended as the base unit, but a one-gram standard is too light to be a practical reference: a speck of dust or a fingerprint changes it measurably. The thousandfold-larger kilogram was made the standard artefact instead, and the SI inherited the arrangement, leaving the base unit as the only one carrying a prefix in its own name.
Cooking outside the English-speaking world runs on grams, and increasingly inside it as well. Weighing flour rather than measuring it by volume removes the largest source of error in baking, because a cup of flour can vary by twenty per cent depending on how it was packed. Professional bakers work entirely in grams and express recipes as percentages of flour weight.
Nutrition panels report protein, fat, carbohydrate, sugar and fibre in grams, so the unit is the everyday currency of dietary arithmetic. Postal services price letters and small parcels in grams, and precious metals, though traded in troy ounces, are sold to the public in gram bars.
It is also the unit the SI prefixes attach to, which is why the system has a base unit that already carries a prefix. Smaller and larger masses are named from the gram and never from the kilogram: a thousandth of a gram is a milligram, not a microkilogram, and a million grams is a megagram, better known as the tonne. The rule spares the language a set of double prefixes that would otherwise be unavoidable, but it leaves the kilogram in the odd position of being the only base unit whose name contains one. Postal tariffs show the gram at work in ordinary life: letter rates step at 20, 50 and 100 grams, and a sheet of office paper weighs about five.
One gram equals 0.001 kilograms, one thousand milligrams, or about 0.035 ounces.
The kilogram is the base unit of mass in the International System of Units. Its symbol is kg. It is the only base unit whose name carries a prefix, an inheritance from the early metric system that has never been tidied away.
For more than a century the kilogram was defined by a single object: a cylinder of platinum-iridium held at the International Bureau of Weights and Measures at Sèvres, near Paris. Forty official copies were distributed to member states and compared with the prototype at long intervals. Those comparisons revealed a slow drift of several tens of micrograms between the prototype and its copies over a hundred years, and there was no way to tell which had changed. A unit that could only be defined by an object no one could check was an embarrassment for a system built on reproducibility.
The redefinition took effect on 20 May 2019. The kilogram is now fixed by assigning an exact value to the Planck constant, 6.62607015 times ten to the power minus thirty-four joule seconds, which ties mass to time and length through quantum mechanics. In practice the realisation is made with a Kibble balance, which weighs a mass against an electromagnetic force whose magnitude is known in terms of electrical quantities, or by counting the atoms in an almost perfect silicon sphere. Any properly equipped laboratory can now realise the unit for itself.
Everyday life is unaffected by any of this. Body weight, groceries, luggage allowances, freight and the mass side of nearly every engineering calculation are all in kilograms, and the international prototype still sits in its vault as a historical object rather than a definition.
Realising the new definition takes an instrument rather than an artefact. A Kibble balance holds a mass against an electromagnetic force whose value is known from the Planck constant, comparing mechanical and electrical power directly, and a second method counts the atoms in an almost perfect sphere of silicon-28 whose diameter and lattice spacing are known to a few parts in a billion. The two approaches agree to within about twenty micrograms in a kilogram, and any laboratory that builds one can now make its own primary standard. That is the practical gain of the change: mass no longer has to be traced back to a single cylinder in a vault, and no accident to that cylinder can alter the unit.
One kilogram equals 1000 grams, approximately 2.20462 pounds, or about 35.274 ounces.