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.
| Unit | Symbol | 1 Microgram equals | 1 of these equals | Converter |
|---|---|---|---|---|
| Nanogram | ng | 1000 ng | 0.001 μg | Micrograms to Nanograms |
| Milligram | mg | 0.001 mg | 1000 μg | Micrograms to Milligrams |
| Gram | g | 0.000001 g | 1000000 μg | Micrograms to Grams |
| Kilogram | kg | 0.000000001 kg | 1000000000 μg | Micrograms to Kilograms |
| Tonne | t | 1 × 10-12 t | 1000000000000 μg | Micrograms to Tonnes |
| Grain | gr | 0.0000154323583529 gr | 64798.91 μg | Micrograms to Grains |
| Dram | dr | 0.000000564383391193 dr | 1771845.19531 μg | Micrograms to Drams |
| Ounce | oz | 0.0000000352739619496 oz | 28349523.125 μg | Micrograms to Ounces |
| Pound | lb | 0.00000000220462262185 lb | 453592370 μg | Micrograms to Pounds |
| Stone | st | 1.57473044418 × 10-10 st | 6350293180 μg | Micrograms to Stones |
| Hundredweight | cwt | 1.96841305522 × 10-11 cwt | 50802345440 μg | Micrograms to Hundredweights |
| Long ton | t | 9.84206527611 × 10-13 t | 1016046908800 μg | Micrograms to Long tons |