| Micrograms (μg) | Pounds (lb) |
|---|---|
| 1 Microgram | 0.00000000220462262185 lb |
| 2 Micrograms | 0.0000000044092452437 lb |
| 3 Micrograms | 0.00000000661386786555 lb |
| 4 Micrograms | 0.0000000088184904874 lb |
| 5 Micrograms | 0.0000000110231131092 lb |
| 10 Micrograms | 0.0000000220462262185 lb |
| 20 Micrograms | 0.000000044092452437 lb |
| 25 Micrograms | 0.0000000551155655462 lb |
| 50 Micrograms | 0.000000110231131092 lb |
| 100 Micrograms | 0.000000220462262185 lb |
| Reference | Micrograms (μg) | Pounds (lb) |
|---|---|---|
| A sheet of A4 paper (80 gsm) | 5000000 μg | 0.0110231 lb |
| One litre of water | 1 × 109 μg | 2.20462 lb |
| Average adult human | 7 × 1010 μg | 154.324 lb |
| A small car | 1.4 × 1012 μg | 3086.47 lb |
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.
The pound is a unit of mass equal to exactly 0.45359237 kilograms. Its symbol is lb, from the Latin libra, and the plural is sometimes written lbs. It is the principal unit of mass in the United States and remains widely used in the United Kingdom for body weight and food.
That exact value dates from the International Yard and Pound Agreement of 1959, in which the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa agreed a single definition for both units. Before it, the American and British pounds differed slightly, which was tolerable in commerce and unacceptable in engineering as aircraft and munitions were built to shared drawings. Defining the pound in terms of the kilogram, rather than the other way round, quietly made the imperial system a derivative of the metric one.
The abbreviation lb is a reminder of the Roman libra pondo, a pound by weight, from which English took the word pound and the symbol from different halves of the phrase. The same libra gave the pound sterling its name and its old symbol, which is why a currency and a weight share a word.
Everyday American life is dense with the unit. Body weight, groceries sold loose, meat, flour and sugar, freight, and gym equipment are all in pounds. Weight-class sports set limits in pounds, and infant birth weights are given in pounds and ounces. In the United Kingdom the pound survives socially rather than commercially: shops sell in kilograms while people still describe their own weight in stones and pounds.
A second pound causes endless trouble in engineering. The pound-force is the weight of one pound of mass in standard gravity, about 4.448 newtons, and it is written with the same abbreviation in most American practice. Since force equals mass times acceleration, using the two interchangeably requires a conversion constant, and generations of textbooks have handled that either by introducing the slug — the mass that one pound-force accelerates at one foot per second squared, about 14.59 kilograms — or by carrying a factor of 32.174 through every calculation. Neither is elegant, and the confusion is the strongest practical argument the metric system has ever had in its favour.
One pound equals 0.45359237 kilograms exactly, 16 ounces, or 7000 grains.