| Reference | Drams (dr) | Kilograms (kg) |
|---|---|---|
| A sheet of A4 paper (80 gsm) | 2.82192 dr | 0.005 kg |
| One litre of water | 564.383 dr | 1 kg |
| Average adult human | 39506.8 dr | 70 kg |
| A small car | 790137 dr | 1400 kg |
The dram is a unit of mass equal to one sixteenth of an avoirdupois ounce, or about 1.7718 grams. Its symbol is dr. There are 256 drams in a pound and 27.34375 grains in a dram, a set of numbers that shows how far the avoirdupois system drifted from tidy arithmetic.
The name descends from the Greek drachma through Latin and Old French, and the same root gives the modern currency of Armenia and the historical coin of Athens. The apothecaries' dram was a different and larger unit, one eighth of a troy ounce or 3.888 grams, and pharmacists used it until the twentieth century. Recipes and old medical texts therefore need reading with care, since an unqualified dram may be either.
Modern survival is narrow but real. Shotgun shells are the best-known case: the dram equivalent printed on a box of cartridges is a legacy figure describing the propellant charge in terms of the black powder that would have produced the same velocity, not the actual weight of modern smokeless powder. American recipes occasionally call for drams, and the unit persists in some descriptions of small hardware and fishing weights.
For everyday purposes the dram is now a curiosity, kept alive by its appearance on packaging and in older texts rather than by any working need. Where precision matters the same quantity is stated in grams, and where tradition matters the word survives without its arithmetic being examined too closely.
Two other drams complicate the picture. The apothecaries' dram, used in pharmacy until the twentieth century, was one eighth of a troy ounce and weighed about 3.888 grams, more than twice the avoirdupois dram described here. The fluid dram is a measure of volume rather than mass, one eighth of a fluid ounce, and it still appears on the graduations of old medicine glasses and on some American pharmacy equipment. A recipe or a formula written before the metric era therefore has to be read with its system identified first, because the same word can mean two different masses and a volume, and nothing in the notation distinguishes them.
One dram equals about 1.7718 grams, one sixteenth of an ounce, or 27.34375 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.