| Micrograms (μg) | Kilograms (kg) |
|---|---|
| 1 Microgram | 0.000000001 kg |
| 2 Micrograms | 0.000000002 kg |
| 3 Micrograms | 0.000000003 kg |
| 4 Micrograms | 0.000000004 kg |
| 5 Micrograms | 0.000000005 kg |
| 10 Micrograms | 0.00000001 kg |
| 20 Micrograms | 0.00000002 kg |
| 25 Micrograms | 0.000000025 kg |
| 50 Micrograms | 0.00000005 kg |
| 100 Micrograms | 0.0000001 kg |
| Reference | Micrograms (μg) | Kilograms (kg) |
|---|---|---|
| A sheet of A4 paper (80 gsm) | 5000000 μg | 0.005 kg |
| One litre of water | 1 × 109 μg | 1 kg |
| Average adult human | 7 × 1010 μg | 70 kg |
| A small car | 1.4 × 1012 μg | 1400 kg |
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 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.