| Nanograms (ng) | Hundredweights (cwt) |
|---|---|
| 1 Nanogram | 1.96841305522 × 10-14 cwt |
| 2 Nanograms | 3.93682611044 × 10-14 cwt |
| 3 Nanograms | 5.90523916567 × 10-14 cwt |
| 4 Nanograms | 7.87365222089 × 10-14 cwt |
| 5 Nanograms | 9.84206527611 × 10-14 cwt |
| 10 Nanograms | 1.96841305522 × 10-13 cwt |
| 20 Nanograms | 3.93682611044 × 10-13 cwt |
| 25 Nanograms | 4.92103263806 × 10-13 cwt |
| 50 Nanograms | 9.84206527611 × 10-13 cwt |
| 100 Nanograms | 1.96841305522 × 10-12 cwt |
| Reference | Nanograms (ng) | Hundredweights (cwt) |
|---|---|---|
| A sheet of A4 paper (80 gsm) | 5 × 109 ng | 0.0000984207 cwt |
| One litre of water | 1 × 1012 ng | 0.0196841 cwt |
| Average adult human | 7 × 1013 ng | 1.37789 cwt |
| A small car | 1.4 × 1015 ng | 27.5578 cwt |
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.
The hundredweight is a unit of mass whose value depends on which side of the Atlantic you are standing. The imperial or long hundredweight is 112 pounds, about 50.802 kilograms. The US or short hundredweight is 100 pounds, about 45.359 kilograms. Its symbol is cwt, from the Latin centum and the English weight.
The name promises a hundred and the imperial version delivers a hundred and twelve, which looks like an error until the arithmetic behind it is seen. Twenty hundredweight make a long ton of 2240 pounds, and a hundredweight is eight stone of fourteen pounds each. The chain of halvings and doublings that runs from the pound to the ton only closes neatly if the intermediate unit is 112, so the number is a consequence of the system rather than a mistake in it.
Its working life was in bulk trade. Coal, cement, grain, potatoes and fertiliser moved by the hundredweight, and sack sizes were built around it: the standard British coal sack held one hundredweight, which is why a coal merchant's cart was rated in a number of hundredweight rather than in tons. Vans and light trucks were once classified the same way, and the phrase survives in some vehicle names.
In the United States the short hundredweight is still used in agricultural markets, where livestock, grain and milk prices are quoted per hundredweight rather than per pound or per ton. Elsewhere the unit has largely given way to the tonne and the kilogram, and its main modern role is to make old records intelligible.
Continental Europe kept a cousin of it under a different name. The quintal, from the Arabic qintar by way of medieval Latin, served the same purpose in French, Spanish, Italian and Portuguese trade, and when the metric system arrived it was simply redefined as a round 100 kilograms. That metric quintal is still the unit in which grain harvests are reported across much of southern Europe, Latin America and India, and it sits close enough to the imperial hundredweight of 50.8 kilograms that old and new figures can look deceptively similar. The parallel is instructive: both units survived because a sack a strong worker could handle is a more natural quantity than any round metric mass.
One imperial hundredweight equals 112 pounds, about 50.802 kilograms, or 8 stone.