| Reference | Drams (dr) | Nanograms (ng) |
|---|---|---|
| A sheet of A4 paper (80 gsm) | 2.82192 dr | 5 × 109 ng |
| One litre of water | 564.383 dr | 1 × 1012 ng |
| Average adult human | 39506.8 dr | 7 × 1013 ng |
| A small car | 790137 dr | 1.4 × 1015 ng |
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 nanogram is a unit of mass equal to one billionth of a gram, or a thousandth of a microgram. Its symbol is ng. Expressed in the base unit of the SI it is 10 raised to the power minus twelve kilograms.
Nothing historical stands behind the nanogram. It exists because SI prefixes extend without limit, and it became useful only when instruments grew sensitive enough to justify it. A good analytical balance resolves tenths of a microgram and stops there; no balance weighs a nanogram directly. The quantity is instead inferred, from the signal a mass spectrometer, an immunoassay or a chromatography column produces when a known volume of sample passes through it.
Clinical laboratories are where most people meet the unit without noticing. Hormone and drug concentrations in blood are reported in nanograms per millilitre: testosterone, prostate-specific antigen, digoxin, tacrolimus, and vitamin D in American practice. Therapeutic drug monitoring depends on the scale, because the difference between an effective and a toxic concentration of some drugs is a few nanograms per millilitre. Anti-doping thresholds are written the same way, which is why an athlete can test positive for a substance present in quantities invisible by any other measure.
Environmental chemistry uses it for the most toxic contaminants, where the interesting concentrations are far below anything a gram-scale unit describes comfortably. Dioxins and furans in air are reported in nanograms per cubic metre, mercury and polycyclic aromatic hydrocarbons in nanograms per litre of water. Regulatory limits for these substances are set at concentrations that only became measurable in the second half of the twentieth century, so the unit and the regulation arrived together.
The scale is hard to picture. A single grain of fine table salt weighs roughly sixty micrograms, which is sixty thousand nanograms. A typical mammalian cell weighs on the order of one nanogram, so a nanogram of tissue is a handful of cells. A microgram of anything is already invisible; a nanogram is a thousandth of that.
One nanogram equals 0.001 micrograms, one millionth of a milligram, and one billionth of a gram.