| Reference | Grains (gr) | Nanograms (ng) |
|---|---|---|
| A sheet of A4 paper (80 gsm) | 77.1618 gr | 5 × 109 ng |
| One litre of water | 15432.4 gr | 1 × 1012 ng |
| Average adult human | 1080265 gr | 7 × 1013 ng |
| A small car | 21605302 gr | 1.4 × 1015 ng |
The grain is a unit of mass equal to exactly 64.79891 milligrams. Its symbol is gr. It is the smallest unit in the imperial and US customary systems, and the only one shared identically by the avoirdupois, troy and apothecaries systems, which makes it the hinge on which conversions between them turn.
Its origin is agricultural and literal: a grain of barley, or in some traditions wheat, taken as a natural standard because seeds from a single crop are remarkably uniform. The English statute of 1266 defined the penny as the weight of thirty-two wheat grains taken from the middle of the ear. That definition survived in principle until the nineteenth century, when the grain was fixed by law against the pound rather than against any seed. Seven thousand grains make an avoirdupois pound, and 5760 make a troy pound.
Firearms are where the unit is most alive today. Bullet mass is quoted in grains almost universally, so a 55-grain rifle bullet or a 230-grain pistol bullet is standard language among shooters worldwide, including in metric countries. Propellant charges are weighed in grains as well, and reloading scales are calibrated in them.
Pharmacy used the grain for centuries, and the five-grain aspirin tablet is the ancestor of today's 325-milligram tablet, a figure that looks arbitrary in metric units and obvious in the old ones. Dentistry, arrow and archery components, and the pearl and gemstone trades all retain it in places.
Its most remarkable property is that it is the same in every English weight system. Troy, avoirdupois and apothecaries' weights each divide differently above the grain — a troy pound is 5760 grains and an avoirdupois pound 7000 — but all three rest on the identical grain of 64.79891 milligrams. That made the grain the common currency by which a goldsmith's weights and a grocer's weights could be compared, and it is the reason the unit survived standardisation while its larger relatives were pruned away. Handloaders exploit the same precision today: powder charges are weighed to a tenth of a grain, about six milligrams, because a small error in charge produces a large change in pressure.
One grain equals 64.79891 milligrams exactly, one seven-thousandth of an avoirdupois pound, or about 0.002286 ounces.
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.