mcg to mg: Micrograms vs Milligrams

mcg vs mg: Micrograms and Milligrams Explained

Key points

There are 1,000 micrograms (mcg) in one milligram (mg). To convert mcg to mg, divide by 1,000. To convert mg to mcg, multiply by 1,000.
"mcg", "µg" and "ug" are three ways of writing the same unit: the microgram, one millionth of a gram.
UK food supplement labels give each listed vitamin and mineral in a unit set in law, either mg or µg, so one label can use both.
International units (IU) are not a unit of mass. Each substance has its own conversion factor, so there is no single IU to mcg number.

One milligram equals 1,000 micrograms, so 500mcg is 0.5mg and 2mg is 2,000mcg. Both units measure mass, and both are fractions of a gram. This guide explains the symbols, works through conversions in both directions and shows why a single label can switch between the two.

What the prefixes mean

The "milli" in milligram and the "micro" in microgram are prefixes from the International System of Units (SI). The UK's National Physical Laboratory (NPL) lists milli, symbol m, as 10⁻³ (0.001, one thousandth) and micro, symbol µ, as 10⁻⁶ (0.000 001, one millionth).

Put a prefix in front of "gram" and you get a smaller unit of mass. Each step down the scale below is 1,000 times smaller than the one above it:

Unit	Symbols you may see	In grams	In milligrams
Gram	g	1	1,000
Milligram	mg	0.001	1
Microgram	mcg, µg, ug	0.000001	0.001

mcg, µg and ug: one unit, three spellings

The SI symbol for micro is the Greek letter µ (mu), so the formal symbol for a microgram is µg. The same unit is also written "mcg", and is sometimes typed "ug" where the µ symbol is not available. The US Food and Drug Administration (FDA) uses "mcg" in its labeling guidance. All three mean exactly the same amount.

µg and mg differ by one character on the page but by a factor of 1,000 in amount. The prescription-writing guidance in the British National Formulary (BNF) says "micrograms" should not be abbreviated.

How to convert mcg to mg, step by step

The numbers below are arithmetic examples only. They are not amounts for anyone to take.

mcg to mg: divide by 1,000

Move the decimal point three places to the left.

1,000mcg = 1mg
500mcg = 0.5mg
250mcg = 0.25mg
5mcg = 0.005mg

mg to mcg: multiply by 1,000

Move the decimal point three places to the right.

0.4mg = 400mcg
2mg = 2,000mcg
0.05mg = 50mcg

A quick check

The same amount always has a bigger number in micrograms than in milligrams, because the microgram is the smaller unit. If a conversion gives you fewer micrograms than milligrams, the decimal point has moved the wrong way.

Why one label can use both units

On a UK food supplement, the brand does not pick the unit for a vitamin or mineral. The Food Supplements (England) Regulations 2003 require the amount of each listed vitamin or mineral to be given in numerical form, using the unit specified in brackets after its name in the list the regulations refer to. Annex XIII of Regulation (EU) No 1169/2011, the food information rules as they apply in Great Britain, shows units in the same way. There, vitamin C, niacin and iron are listed in mg, while vitamin D, vitamin B12, folic acid and iodine are listed in µg. So a single multivitamin label can show some lines in mg and others in µg.

The same annex holds the nutrient reference values behind the "%NRV" column on a label. That column is explained in our guide What Does NRV Mean on a Supplement Label?

BNF guidance on writing prescriptions follows a similar pattern. Quantities below 1 gram are written in milligrams (500 mg, not 0.5 g), and quantities below 1 milligram are written in micrograms (100 micrograms, not 0.1 mg). In both examples, the number is written without a leading decimal point.

International units (IU)

Some labels show certain vitamins in international units, written IU. An IU is not a weight. FDA guidance describes the IU used for vitamin A as a measurement based on biological activity, and it sets out separate conversion factors for different forms of vitamin A.

Each substance therefore has its own factor. For vitamin D, the FDA gives 1 mcg of cholecalciferol as equal to 40 IU. FDA guidance for US labels says vitamin D is declared in mcg, and that IU may still appear in brackets beside the mcg figure. A conversion that is correct for one vitamin will be wrong for another, so check which substance a figure refers to before converting it.

Cobalt glass bottle shoulder on a black background

Reading the units on any label

A few habits make unit mix-ups less likely:

Read the unit before the number. 50mg and 50µg are 1,000 times apart.
Check whether a figure is for the whole pack or for a stated amount of the product. They are different figures, even when the unit is the same.
Convert everything to one unit before comparing two products.
If a figure on a medicine or supplement looks wrong by a factor of 1,000, ask a pharmacist before using it.

For other things to look at on a pack, read How to read a supplement label properly.

The figure on our own bottle is a whole-bottle figure in milligrams: 3000mg of NAD+ in total per 30ml bottle. It is not the amount in a single serving. For what NAD+ is as a molecule, see the science page.

Pure Limitless sells one product: NAD+ Sublingual Drops, a food supplement with 3000mg of NAD+ in total per 30ml bottle.

Frequently asked questions

How many micrograms are in a milligram?

1,000. A milligram is one thousandth of a gram and a microgram is one millionth of a gram, so there are 1,000 micrograms in each milligram.

Are mcg and µg the same?

Yes. µg is the SI symbol for microgram, and mcg is a common alternative that avoids the Greek letter. "ug" is the same unit typed without the µ.

Can IU be converted to mcg with one number?

No. An IU is based on a substance's activity, not its weight, so the factor differs from one vitamin to another. The FDA, for example, gives 1 mcg of vitamin D (cholecalciferol) as 40 IU and uses other factors for vitamin A.

Sources

By Pure Limitless

Sep 30, 2026

0 Comment

Labels and Quality