| Nanograms (ng) | Kilograms (kg) |
|---|---|
| 1 Nanogram | 1 × 10-12 kg |
| 2 Nanograms | 2 × 10-12 kg |
| 3 Nanograms | 3 × 10-12 kg |
| 4 Nanograms | 4 × 10-12 kg |
| 5 Nanograms | 5 × 10-12 kg |
| 10 Nanograms | 1 × 10-11 kg |
| 20 Nanograms | 2 × 10-11 kg |
| 25 Nanograms | 2.5 × 10-11 kg |
| 50 Nanograms | 5 × 10-11 kg |
| 100 Nanograms | 1 × 10-10 kg |
| Reference | Nanograms (ng) | Kilograms (kg) |
|---|---|---|
| A sheet of A4 paper (80 gsm) | 5 × 109 ng | 0.005 kg |
| One litre of water | 1 × 1012 ng | 1 kg |
| Average adult human | 7 × 1013 ng | 70 kg |
| A small car | 1.4 × 1015 ng | 1400 kg |
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 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.