What Is Mcg?
What is mcg? It is the short form of microgram, a unit of mass. One microgram is one millionth of a gram, or one thousandth of a milligram.
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Mcg means microgram
The word has two parts. "Micro" is a metric prefix that means one millionth. "Gram" is the unit. Put them together and you get one millionth of a gram.
NIST, the US measurement lab, lists micro in its table of SI prefixes. Its official symbol is the Greek letter mu, written µ. So the official way to write microgram is µg.
| Unit | Short form | Part of a gram |
|---|---|---|
| gram | g | 1 |
| milligram | mg | 0.001 (one thousandth) |
| microgram | mcg or µg | 0.000001 (one millionth) |
| nanogram | ng | 0.000000001 (one billionth) |
Each step down the table is 1,000 times smaller. To switch between mg and mcg, use the mg to mcg converter. For the step below, see ng to mcg.
Three ways to write it: mcg, µg and ug
All three mean the same unit. They show up in different places.
| Spelling | Where you see it | Why |
|---|---|---|
| µg | science papers, the SI system | It is the official symbol. |
| mcg | US labels, medicine and pharmacy | A safety group asks for it, because µg can be misread. |
| ug | plain text, emails and some websites | It is a stand in when the µ key is hard to find. |
The safety group is the Institute for Safe Medication Practices (ISMP). Its list of error prone symbols says µg is "mistaken as mg" and tells people to write mcg. A handwritten µ can look like an m. A mix up like that is 1,000 times off.
US food and supplement labels follow the same habit. The FDA writes micrograms as mcg in its label rules.
How small is a microgram?
Very small, because it sits two steps below a gram. A gram is 1,000 milligrams and a milligram is 1,000 micrograms, so one gram holds a million micrograms.
Here is the same amount written three ways:
- 0.000001 g
- 0.001 mg
- 1 mcg
That is why small amounts are often written in mcg. "50 mcg" is easier to read than "0.05 mg", and it has no decimal point to miss. To see how the two units compare, read mg vs mcg.
What mcg RAE and mcg DFE mean
On a US Supplement Facts label you may see letters after mcg:
- mcg RAE is used for vitamin A. RAE stands for retinol activity equivalents.
- mcg DFE is used for folate. DFE stands for dietary folate equivalents.
Both are still micrograms. The extra letters show that the FDA's label rules adjust the number for the form of the nutrient. The supplement label guide explains them in more detail.
How to type the µ sign
Most people never need to. Writing mcg is clear and is what labels use. If you do need the symbol:
- On a Mac, press Option and M.
- On Windows, hold Alt and type 0181 on the number pad.
- On any device, you can copy it from this page: µ
Questions
What does mcg stand for?
Mcg stands for microgram. It is one millionth of a gram, and 1,000 mcg make 1 mg.
What is the symbol for microgram?
The official symbol is µg, from the Greek letter mu. Labels in the US write mcg instead, because µg can be misread as mg.
Is ug the same as mcg?
Yes. Ug, mcg and µg all mean microgram. Ug is often typed when the µ sign is hard to reach.
What is a microgram?
A microgram is a metric unit of mass. It is one millionth of a gram, so 1,000,000 mcg make 1 g.
How do you type the micro symbol?
On a Mac, press Option and M. On Windows, hold Alt and type 0181 on the number pad. You can also just write mcg.
How small is a microgram?
It is 0.001 mg, or 0.000001 g. A single milligram holds 1,000 of them.
Sources
- The International System of Units (SI), NIST Special Publication 330, 2019 edition, Table 7 SI prefixes and Table 8 non-SI units such as the liter, National Institute of Standards and Technology (NIST), doi:10.6028/NIST.SP.330-2019, read 2026-10-04
- Metric (SI) Prefixes, National Institute of Standards and Technology (NIST), read 2026-10-04
- ISMP List of Error-Prone Abbreviations, Symbols, and Dose Designations, Institute for Safe Medication Practices, 2021, read 2026-10-04
- Converting Units of Measure for Folate, Niacin, and Vitamins A, D, and E on the Nutrition and Supplement Facts Labels, US Food and Drug Administration, August 2019, read 2026-10-04