Mcg to Mg Converter
Convert micrograms to milligrams — the rung where a misread symbol multiplies by a thousand.
Updated
milligrams
1.0000mg
1,000 mcg = 1.0000 mg
Advertisement
In short
How many micrograms are in a milligram?
Exactly 1,000, so divide micrograms by 1,000 to get milligrams: 500 mcg is 0.5 mg and 400 mcg is 0.4 mg. The division is trivial. The reason this conversion has its own safety literature is that mcg and mg look alike once written by hand.
Micrograms measure mass. International Units, printed on many vitamin labels, are not a mass unit and do not convert without a substance-specific factor.
micrograms to milligrams — quick reference
| micrograms (mcg) | milligrams (mg) |
|---|---|
| 25 mcg | 0.0250 mg |
| 50 mcg | 0.0500 mg |
| 100 mcg | 0.1000 mg |
| 200 mcg | 0.2000 mg |
| 400 mcg | 0.4000 mg |
| 500 mcg | 0.5000 mg |
| 1,000 mcg | 1.0000 mg |
| 5,000 mcg | 5.0000 mg |
The formula, worked line by line
One division by an exact power of ten. Micro- means one millionth by definition and milli- one thousandth, so a microgram is exactly a thousandth of a milligram. No measurement was involved in fixing that relationship and no better factor exists to be found later.
“Use mcg instead of the Greek mu symbol for microgram, which has been mistaken for mg and produced thousandfold overdoses.”
Notice what that guidance is not saying. It is not that the symbol is wrong — it is the correct SI symbol. It is that correctness and safety are different properties, and where they conflict on a handwritten chart, the field chose safety.
milligrams = micrograms / 1,000
micro- = 10^-6, milli- = 10^-3
400 mcg / 1,000 = 0.4 mg
1 mg = 1,000 mcg, and 1 g = 1,000,000 mcg
IU has no general conversion to mcg- Start
- 750 mcg
- Move three places left
- 0.75 mg
- Move three more
- 0.00075 g
- Move three the wrong way
- 750,000 mg
- Each rung
- three decimal places, one factor of 1,000
The final row is the direction error rather than the prefix error, and it is the reason a converter is worth using even for arithmetic this simple: the machine cannot divide when it meant to multiply.
Why the number on the label is usually the bigger one
Given a free choice between 400 mcg and 0.4 mg, labels overwhelmingly print the first. A larger number with no decimal point is harder to corrupt than a smaller one with a leading decimal, and the risk being managed is not the reader misjudging the size, it is the mark disappearing.
Where the microgram sits in the full ladder
- 1 microgram (mcg)
- 0.001 mg
- 1 milligram (mg)
- 1,000 mcg
- 1 gram (g)
- 1,000,000 mcg
- 1 kilogram (kg)
- 1,000,000,000 mcg
The uniformity of the ladder is what makes it easy to climb and what makes a slip at any rung cost exactly the same as a slip at any other.
How to use the mcg to mg converter
Divide by a thousand. Micro- is defined as exactly one millionth and milli- as exactly one thousandth, so the step between them is a clean factor of 1,000 with nothing to round. As arithmetic this page is finished in one line, and everything below is about the part that actually goes wrong.
What goes wrong is reading, not dividing. A quantity written 400 mcg and the same quantity written 0.4 mg are identical, but they fail in opposite ways: the first fails if the prefix is misread, the second fails if the decimal point is lost. Choosing between those two failure modes is what the unit on a label is doing.
1,000
Micrograms in a milligram
micro- is one millionth
400 mcg
Is 0.4 mg
no decimal point to lose
1,000x
The cost of misreading it
not a small overshoot
Advertisement
This is why so many strengths are printed in micrograms even though milligrams would be a smaller number to say. A figure with no decimal point cannot lose one. Levothyroxine at 100 mcg, folic acid at 400 mcg and vitamin B12 at 1,000 mcg could all be written in milligrams, and every one of them would then carry a leading decimal.
The other half of the problem is the symbol itself. In SI the microgram is written with the Greek letter mu, as µg. Handwritten, a mu can be read as an m — and mg is a real unit sitting one rung up, so the misreading produces a plausible answer rather than nonsense. That is the property that makes it dangerous.
- 400 mcg
- the form a label prints
- 0.4 mg
- correct, but carries a decimal
- .4 mg
- a tenfold error waiting for a smudge
- 400 µg
- correct SI, discouraged in handwriting
- 400 mg
- what a misread mu produces
Four of these five rows are the same amount of folic acid. The last one is a thousand times more, and it differs from the fourth by a single stroke of a pen.
Go one rung further up the ladder
Milligrams to grams is the same division again, and that page covers the leading-zero and trailing-zero rules in full.
Open the mg to g converter →Micrograms converted to milligrams, with the strengths that commonly appear on labels marked in the third column.
| Micrograms | Milligrams | Where the figure turns up |
|---|---|---|
| 25 mcg | 0.025 mg | Vitamin D, stated as 1,000 IU |
| 50 mcg | 0.05 mg | A common levothyroxine strength |
| 100 mcg | 0.1 mg | A common levothyroxine strength |
| 200 mcg | 0.2 mg | Selenium on many supplement panels |
| 400 mcg | 0.4 mg | The folic acid figure on prenatal labels |
| 500 mcg | 0.5 mg | Half a milligram, printed without a decimal |
| 600 mcg | 0.6 mg | A folate figure in dietary guidance |
| 800 mcg | 0.8 mg | A higher folic acid strength |
| 1,000 mcg | 1 mg | Vitamin B12 tablets |
| 2,000 mcg | 2 mg | Two milligrams, rarely written this way |
| 5,000 mcg | 5 mg | Five milligrams, always written as mg |
Advertisement
Why medicine writes mcg rather than the SI symbol
The microgram has a correct international symbol, µg, and medication safety guidance asks people not to use it. That is an unusual thing for a field to decide. Most disciplines treat SI notation as the thing you standardise on; here the standard notation is the hazard, and a non-standard abbreviation was adopted deliberately in its place.
The reasoning is legibility under bad conditions. A printed µ is unmistakable. A handwritten one, photocopied, faxed, or read at speed, is a lowercase m with a tail — and mg is the unit one thousandfold away. The letters m, c and g share no such confusion, which is the entire argument for mcg.
Do
- Write mcg, or spell out microgram in full
- Keep a strength under 1 mg in micrograms where the label does
- Read the unit before you read the number
- Treat a printed µg on packaging as correct and clear
Don't
- Use the Greek mu symbol in anything handwritten
- Rewrite 400 mcg as .4 mg to look tidier
- Assume a bare number carries the unit you expect
- Convert International Units as though they were micrograms
Note the asymmetry in that list. Printed µg on a manufactured package is fine and appears on plenty of them. The guidance is about what happens when a human writes the symbol down, which is where a defined mark stops being defined and becomes handwriting.
International Units are not micrograms
Open a vitamin D bottle and the strength may read 1,000 IU rather than 25 mcg. An International Unit is a measure of biological activity, agreed separately for each substance, and it is not a mass at all. There is no general IU-to-microgram factor, and a converter that offered one would be inventing it.
- Vitamin D
- 1 IU is 0.025 mcg, so 1,000 IU is 25 mcg
- Vitamin E, natural form
- 1 IU is about 0.67 mg of d-alpha-tocopherol
- Vitamin E, synthetic form
- 1 IU is about 0.45 mg of dl-alpha-tocopherol
- The general factor
- does not exist
- The rule
- IU converts per substance, and per form
Vitamin E needs two different factors for two forms of the same vitamin. That is the clearest demonstration that IU measures activity rather than matter, and why this tool converts mass only.
The practical consequence is small but worth stating. If a label gives IU, the number you want is usually printed next to it in micrograms already, because that dual labelling has been standard on nutrition panels for years. Read it off rather than deriving it.
Every micrograms value, worked out
45 common micrograms figures each get their own page, with the answer at full precision, the arithmetic, what rounding costs, and the nearest real-world reference point on the scale.
Advertisement
Questions people ask
How many mcg are in 1 mg?
Exactly 1,000, because micro- is defined as one millionth and milli- as one thousandth. So 500 mcg is 0.5 mg, 250 mcg is 0.25 mg, and 1,000 mcg is 1 mg. The relationship is a definition rather than a measurement, so it carries no uncertainty at all.
Is mcg the same as µg?
They are the same unit written two ways. The Greek mu symbol is the correct SI form, and mcg is the abbreviation medication safety guidance asks for instead. The reason is handwriting: a mu can be read as an m, which turns a microgram into a milligram and multiplies the quantity by a thousand.
Is 1000 mcg the same as 1 mg?
Yes, exactly. A 1,000 mcg vitamin B12 tablet and a 1 mg one contain the same amount. Labels tend to print the microgram figure because 1,000 has no decimal point in it, whereas smaller strengths written in milligrams would need one and a decimal point can be lost.
How do I convert IU to mcg?
Only with a factor specific to that substance, because an International Unit measures biological activity rather than mass. For vitamin D, 1 IU is 0.025 mcg, so 1,000 IU is 25 mcg. Vitamin E needs a different factor again, and a different one for each of its two forms. No general conversion exists.
Why is levothyroxine dosed in micrograms?
Because the strengths involved sit below one milligram, and printing them in micrograms avoids a leading decimal. A 100 mcg tablet written as 0.1 mg is the same dose, but the second form fails badly if the decimal point is missed, and the first form has nothing to miss.
Sources
Where the constants and formulas on this page come from. Each line names the figure it backs.
SI prefixes are exact powers of ten, with micro- defined as one millionth and milli- as one thousandth.
The International System of Units (SI), 9th edition — Bureau International des Poids et Mesures
The Greek mu symbol for microgram is on the published do-not-use list because it has been misread as mg, and mcg is the recommended alternative.
List of Error-Prone Abbreviations, Symbols, and Dose Designations — Institute for Safe Medication Practices
Vitamin D is 1 IU per 0.025 micrograms, and vitamin E requires separate IU factors for its natural and synthetic forms.
Vitamin D and Vitamin E Fact Sheets for Health Professionals — NIH Office of Dietary Supplements
Related guides
Unit Converters
How Many Decimals to Keep When Converting Units
The factor is exact to nine digits. The measurement you typed is good to one. Only one of those two limits the answer, and it is not the factor.
August 19, 2026 · 9 min read
Unit Converters
Converting Temperature Differences: Why 10 °C of Change Is 18 °F, Not 50
A day that warms from 10 °C to 20 °C warms by 18 °F, not 50. The 32 belongs to temperatures — a difference never sees it.
August 16, 2026 · 9 min read
Dates & Time
Time and a Half: The Regular Rate Is Not Your Hourly Wage
Every explanation says multiply your hourly rate by 1.5. That is the wrong number the moment a bonus, a shift differential, or a second pay rate enters the week.
August 30, 2026 · 8 min read