| Reference | Ounces (oz) | Kilograms (kg) |
|---|---|---|
| A sheet of A4 paper (80 gsm) | 0.17637 oz | 0.005 kg |
| One litre of water | 35.274 oz | 1 kg |
| Average adult human | 2469.18 oz | 70 kg |
| A small car | 49383.5 oz | 1400 kg |
The ounce is a unit of mass equal to one sixteenth of an avoirdupois pound, or exactly 28.349523125 grams. Its symbol is oz, from the Italian onza. It is the everyday small weight of the United States and remains in common use in the United Kingdom alongside metric units.
There are several ounces, and confusing them is easy. The avoirdupois ounce described here is the one used for food and general goods. The troy ounce, used for gold, silver and platinum, is about 31.103 grams, some ten per cent heavier, which is why a troy pound of twelve ounces weighs less than an avoirdupois pound of sixteen. The fluid ounce is not a mass at all but a volume, and the imperial and US fluid ounces differ from each other by about four per cent.
American food packaging is built on the ounce. Net weights appear in ounces below sixteen and in pounds and ounces above, and recipe quantities for cheese, chocolate, pasta and meat are given the same way. Postal rates for letters step in ounces, and boxing, wrestling and other weight-class sports state limits in pounds and ounces.
The word comes from the Latin uncia, meaning a twelfth part, which is also the ancestor of the inch. That original twelfth survives in the troy ounce but not in the avoirdupois one, whose sixteenth-of-a-pound relationship came later from the wool trade of medieval Europe. Two units with the same name and different arithmetic is the direct result.
Precious metals are the other ounce's territory. Gold, silver, platinum and palladium are priced per troy ounce of 31.1035 grams, about ten per cent heavier than the avoirdupois ounce used for groceries, and every quoted bullion price in the world refers to it. A one-ounce gold coin is therefore heavier than an ounce of butter, and a bar described as a kilogram contains 32.15 troy ounces rather than 35.27 ordinary ones. The troy system also keeps its own subdivisions, with 20 pennyweights to the ounce, which is why a jeweller's scale and a kitchen scale are calibrated differently even when both are marked in ounces.
One ounce equals 28.349523125 grams exactly, one sixteenth of a pound, or 437.5 grains.
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.