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IU to mg Calculator

Some compounds are labelled in international units rather than milligrams, which makes converting between the two the first step before any dosing arithmetic works. The catch is that there is no universal conversion — an IU is a unit of biological activity, not of mass.

Illustrative reference only — nothing here recommends a dose or a potency figure.

Open the IU to mg calculator

Why there is no single IU-to-mg number

An international unit measures how much a substance does, not how much it weighs. It was defined so that different preparations of the same substance — which historically varied in purity — could be compared by effect rather than by mass. That means the milligrams behind one IU depend entirely on the substance and its potency, and a conversion that is right for one compound is meaningless for another.

So every conversion needs one extra number: the potency, expressed as IU per mg. It is printed on the product label or certificate of analysis. This site never assumes a value for it — you supply it, and the arithmetic follows.

Total IU = mg × potency  ·  mg = IU ÷ potency

Worked example

Illustrative numbers only — not a dose recommendation.

A 10 mg vial with a stated potency of 3 IU/mg holds 10 × 3 = 30 IU. Reconstituted with 2 mL of bacteriostatic water, that is 30 IU ÷ 2 mL = 15 IU/mL.

Example
Vial10 mg
Potency3 IU/mg
Total in vial30 IU
Bacteriostatic water2 mL
Concentration15 IU/mL
A 2 IU dose is0.133 mL = 13.3 units

From there the measurement is ordinary reconstitution arithmetic: divide the dose by the concentration for millilitres, then multiply by 100 for units on a U-100 syringe.

Conversions at a few potencies

The table shows how much the answer moves with the potency figure alone — which is exactly why the number has to come off your own label rather than a general reference.

Milligrams equivalent to a given number of IU, at different stated potencies
Potency1 IU2 IU5 IU10 IU
1 IU/mg1 mg2 mg5 mg10 mg
2 IU/mg0.5 mg1 mg2.5 mg5 mg
3 IU/mg0.333 mg0.667 mg1.667 mg3.333 mg
5 IU/mg0.2 mg0.4 mg1 mg2 mg

Frequently asked questions

How do you convert IU to mg?
Divide the number of international units by the potency of that specific substance, stated as IU per mg on its label or certificate of analysis. To go the other way, multiply milligrams by the potency to get IU. There is no universal conversion factor, because an IU measures biological activity rather than mass.
Why does the same IU figure mean different milligrams for different compounds?
Because the international unit is defined separately for each substance, in terms of a reference preparation of that substance. One IU of one compound and one IU of another are equivalent in assigned activity, not in weight, so their mass equivalents are unrelated.
Where do I find the potency of my vial?
It is normally printed on the product label or in the certificate of analysis, expressed as IU per mg or as the total IU in the vial alongside the milligram figure. If only the total IU and the total milligrams are given, dividing one by the other gives the potency.
Do syringe units and international units mean the same thing?
No, and confusing them is a common error. A unit on an insulin syringe is a volume mark — one hundredth of a millilitre on a U-100 syringe. An international unit is a measure of biological activity with no fixed volume. The calculator converts an IU dose into millilitres first, then into syringe units.