Milligram and Microgram Converter

Convert between grams, milligrams and micrograms, with an optional IU conversion using the potency from your own label.

Draft — awaiting review. Not indexed.

Formula: Metric mass units are fixed powers of ten: 1 g = 1,000 mg = 1,000,000 mcg. IU has no universal factor and uses the IU-per-mg figure you supply.

This tool performs mathematical conversions only. It is not medical advice and does not recommend, endorse, or evaluate the safety, efficacy, or legal status of any substance. Consult a licensed healthcare provider before using any medication or peptide, and verify the legal status of any substance in your jurisdiction before use.

What this calculates

This converts between the metric mass units that dose figures are written in, and — separately, and on different terms — to and from International Units. The metric side is straightforward arithmetic: a gram is a thousand milligrams and a milligram is a thousand micrograms, always, for every substance. The IU side is not, and the difference is the most important thing on this page. An International Unit is not a unit of mass at all; it is a unit of biological activity, defined separately for each substance by an international reference preparation. There is no universal number of milligrams in an IU.

How it works

Metric conversions multiply or divide by powers of a thousand, routed through milligrams. For IU, the calculator asks you for the potency — the IU per milligram — printed on your own vial or leaflet, and uses that. It deliberately does not carry a built-in table of conversion factors, because such a table would have to name substances and state their potencies, which is exactly the compound-specific claim this category does not make. Different preparations of the same substance can also carry different potencies, so a figure read off the product in front of you is more reliable than any table would be.

Worked example

Metric first: 0.25 mg is 0.25 × 1,000 = 250 mcg, and 1,500 mcg is 1,500 ÷ 1,000 = 1.5 mg. Now an IU conversion. Suppose a label states a potency of 3 IU per mg — an arbitrary figure used only to show the arithmetic. Then 6 IU is 6 ÷ 3 = 2 mg, and 0.5 mg is 0.5 × 3 = 1.5 IU. Change the potency and every one of those answers changes, which is why the field is required rather than optional.

Common mistakes

Treating IU as though it converts like a metric unit is the significant error, and it is easy to make because IU figures sit alongside mg figures on the same label. They are different kinds of quantity. Second, borrowing a potency figure from a different product or a different manufacturer — preparations vary, and the number you need is the one on the item in your hand. Third, the ordinary decimal slip between mg and mcg: three orders of magnitude is a large distance to travel by misplacing a point, and it is worth reading the result back against the input rather than accepting it.

Common questions

How many mcg are in a mg?

Exactly 1,000, for every substance. That relationship is fixed by the SI prefixes and does not depend on what is being measured.

Why does the IU conversion need a number from me?

Because an IU measures biological activity rather than mass, and it is defined separately for each substance. There is no general mg-per-IU factor, so the only correct source is the potency stated on your own product.