IU vs mg
A milligram is a weight. An international unit (IU) is an amount of biological activity, defined separately for each substance. There is no general IU-to-mg conversion; the factor belongs to the substance and is printed on the product label.
What an IU is
Some medicines and vitamins were first measured by what they did in a standard test, before they could be weighed in pure form. The international unit records that: one IU is the amount of a substance that produces an agreed effect, set against an international reference standard. The World Health Organization maintains those standards for many biological medicines.
The definition is made one substance at a time. One IU of one thing has no relationship to one IU of another.
The factor is different for every substance
| Substance | Approximate relationship | Notes |
|---|---|---|
| Insulin (human) | 1 unit ≈ 0.0347 mg | So U-100 insulin, 100 units/mL, is about 3.47 mg/mL |
| Somatropin (HGH) | 1 mg ≈ 3 IU | Rule of thumb; use the figure on the vial |
| Vitamin D | 1 IU = 0.025 mcg | 40 IU = 1 mcg |
One unit of insulin is about 0.035 mg. One IU of vitamin D is 0.000025 mg. A single conversion factor cannot cover both, and a factor borrowed from one substance and applied to another gives a meaningless number.
The table is there to show how far apart the factors are. For any real calculation, take the figure from your own product's label, because some products state their own potency per mg.
Three things called "units"
Much of the confusion comes from one word doing three jobs.
- International units (IU) of a substance: biological activity, as above.
- Units of insulin: the activity unit for insulin specifically. U-100 insulin has 100 of them per mL.
- Units on an insulin syringe: tick marks, each 0.01 mL on a U-100 barrel. A measure of volume.
For U-100 insulin, the second and third line up by design, which is the whole point of the syringe. For anything else, a syringe unit is 0.01 mL and says nothing about IU or mg. A vial labelled in IU and drawn into an insulin syringe involves two unrelated kinds of unit, and the concentration connects them. Units vs mg vs mL goes through that.
Calculating with a vial labelled in IU
The reconstitution arithmetic does not care whether the amount is in mg or IU, as long as the vial and the prescription use the same one.
For example, a vial labelled 10 IU mixed with 1 mL is 10 IU/mL. Each 10 syringe units (0.1 mL) holds 1 IU. With 2 mL it is 5 IU/mL, and each 10 syringe units holds 0.5 IU. Note that syringe units and IU are different numbers in both cases.
The HGH calculator works in IU for this reason, and the unit converter handles the volume side.
When the vial and the prescription disagree
Sometimes a vial is labelled in mg and the prescription is written in IU, or the reverse. Converting between them needs the factor for that exact product.
- Look on the vial label, carton or package insert. Products measured in IU often state both, for example an amount in mg alongside its IU equivalent.
- If both are not printed, ask the pharmacist. Do not take a factor from a chart for a different product, or from a general rule, and assume it fits.
- Once both figures are in the same unit, the calculation is the ordinary one on the calculator.
Reading a vial label shows where the amount and its unit appear.
IU and mcg, IU and mL
The same reasoning applies to other pairings. IU to mcg is IU to mg with a factor of 1,000 added (see mcg vs mg), and still needs the substance's own factor. IU to mL is a concentration question: it depends on how many IU are in each mL of that particular liquid.
Questions
How many mg is 1 IU?
It depends on the substance. An IU measures biological activity and is defined separately for each one. For example, 1 unit of insulin is about 0.0347 mg, while 1 IU of vitamin D is 0.025 mcg. Use the factor on your product's label.
Is an IU the same as a unit on an insulin syringe?
Only for U-100 insulin. A unit on a U-100 syringe is a volume of 0.01 mL. For any other liquid, the IU in that volume depends on the concentration in IU per mL.
How do I convert HGH from IU to mg?
A common rule of thumb is that 1 mg of somatropin is about 3 IU, but products can differ. Use the figure printed on your vial or package insert, or ask your pharmacist.
Can I use a peptide calculator for a vial labelled in IU?
Yes, if the prescription is also in IU. Divide the IU in the vial by the mL of water for IU/mL, divide the prescribed IU by that for mL, and multiply by 100 for syringe units.