| Reference | Ounces (oz) | Micrograms (μg) |
|---|---|---|
| A sheet of A4 paper (80 gsm) | 0.17637 oz | 5000000 μg |
| One litre of water | 35.274 oz | 1 × 109 μg |
| Average adult human | 2469.18 oz | 7 × 1010 μg |
| A small car | 49383.5 oz | 1.4 × 1012 μg |
The ounce is a unit of mass equal to one sixteenth of an avoirdupois pound, or exactly 28.349523125 grams. Its symbol is oz, from the Italian onza. It is the everyday small weight of the United States and remains in common use in the United Kingdom alongside metric units.
There are several ounces, and confusing them is easy. The avoirdupois ounce described here is the one used for food and general goods. The troy ounce, used for gold, silver and platinum, is about 31.103 grams, some ten per cent heavier, which is why a troy pound of twelve ounces weighs less than an avoirdupois pound of sixteen. The fluid ounce is not a mass at all but a volume, and the imperial and US fluid ounces differ from each other by about four per cent.
American food packaging is built on the ounce. Net weights appear in ounces below sixteen and in pounds and ounces above, and recipe quantities for cheese, chocolate, pasta and meat are given the same way. Postal rates for letters step in ounces, and boxing, wrestling and other weight-class sports state limits in pounds and ounces.
The word comes from the Latin uncia, meaning a twelfth part, which is also the ancestor of the inch. That original twelfth survives in the troy ounce but not in the avoirdupois one, whose sixteenth-of-a-pound relationship came later from the wool trade of medieval Europe. Two units with the same name and different arithmetic is the direct result.
Precious metals are the other ounce's territory. Gold, silver, platinum and palladium are priced per troy ounce of 31.1035 grams, about ten per cent heavier than the avoirdupois ounce used for groceries, and every quoted bullion price in the world refers to it. A one-ounce gold coin is therefore heavier than an ounce of butter, and a bar described as a kilogram contains 32.15 troy ounces rather than 35.27 ordinary ones. The troy system also keeps its own subdivisions, with 20 pennyweights to the ounce, which is why a jeweller's scale and a kitchen scale are calibrated differently even when both are marked in ounces.
One ounce equals 28.349523125 grams exactly, one sixteenth of a pound, or 437.5 grains.
The microgram is a unit of mass equal to one millionth of a gram, or one thousandth of a milligram. Its official symbol is µg, written with the Greek letter mu.
In medicine that symbol is a known hazard. Handwritten, µg is easily read as mg, a thousandfold error, and the mistake has killed patients. Safety bodies including the Institute for Safe Medication Practices therefore recommend writing mcg instead on prescriptions and labels, and most hospital systems now enforce it. The abbreviation is not SI and would be wrong in a scientific paper, but it is the safer choice on a drug chart.
Micronutrients live at this scale. Vitamin B12, folate, iodine, selenium, biotin and vitamin K are all dosed in micrograms, and daily requirements run from a few micrograms to a few hundred. Vitamin D is given either in micrograms or in international units, with one microgram equal to forty IU, a dual convention that causes constant confusion on supplement labels. Potent drugs use the unit too: fentanyl, levothyroxine, digoxin and most hormonal contraceptives are dosed in micrograms rather than milligrams.
Air quality reporting has made the microgram familiar to the public. Fine particulate matter is measured in micrograms per cubic metre, and the World Health Organization guideline for annual PM2.5 exposure is five micrograms per cubic metre. Nitrogen dioxide, sulphur dioxide and ozone limits are written the same way, so the number quoted in a news report about pollution is almost always a count of micrograms in a cubic metre of air.
Toxicology works here because potency and hazard both scale with dose. Digoxin is prescribed in a few hundred micrograms a day and becomes dangerous not far above that, thyroid hormone is dispensed in 25-microgram steps, and the opioid fentanyl is active at doses of tens of micrograms, which is why illicitly mixed powders containing it are so often lethal. Analytical chemistry meets the unit from the other direction: modern instruments detect pesticides, metals and pharmaceuticals in water at concentrations of micrograms per litre, and regulatory limits are written at that level. A microgram is invisible, weighs less than a speck of dust, and yet is the natural unit for both the dose that heals and the dose that kills.
One microgram equals 0.001 milligrams, one millionth of a gram, and one thousand nanograms.