TL;DR: An International Unit is a measure of biological activity, not mass, and it is defined separately for every substance against its own reference standard. That means there is no universal IU-to-mg factor — somatropin runs at roughly 3 IU per mg, hCG at roughly 9,000–10,000 IU per mg. Same three letters on the label, a factor about 3,000× apart. Convert IU to mg using the factor for that compound, then hand the milligrams to the reconstitution calculator for volume and syringe units. Every unit figure below assumes a U-100 syringe, where 1 unit = 0.01 mL.
Why some vials are labelled in units at all
Milligrams are easy to measure and impossible to argue with. So why would anyone label a vial in units?
Because for some substances, mass and effect came apart historically. Early preparations of growth hormone, gonadotropins, insulin and heparin were extracted from tissue, and what you got in a milligram of powder depended on how pure the extraction was. Two vials with identical mass could differ several-fold in what they actually did. The workaround was to stop measuring the powder and start measuring the activity: define a physical reference preparation, assign it an arbitrary unitage by bioassay, and express every subsequent batch as a multiple of that.
That is the International Unit. It is not a metric unit and it does not sit in a conversion table next to grams and litres. It is a per-substance bookkeeping convention, and it survives today mostly by inertia — the labelling stuck even after the compounds became recombinant and perfectly quantifiable by mass.
The factor is never transferable
This is the single point the whole article exists to make. Because each IU is calibrated against its own reference standard, the number of IU in a milligram is a property of that substance — nothing else. Carrying a factor from one compound to another is not an approximation. It is a different question with a different answer.
Here is the spread across substances that commonly carry IU labels:
| Substance | Approx. IU per mg | What 1 IU weighs |
|---|---|---|
| Vitamin E (d-alpha-tocopherol) | ~1.49 | ~0.67 mg |
| Somatropin (rhGH) | ~3 | ~333 mcg |
| Human insulin | ~28.8 | ~34.7 mcg |
| Unfractionated heparin | ~150 | ~6.7 mcg |
| hCG | ~9,000–10,000 | ~0.10–0.11 mcg |
| Vitamin D3 (cholecalciferol) | 40,000 | 0.025 mcg |
Read the right-hand column top to bottom. One IU of vitamin E weighs two-thirds of a milligram. One IU of vitamin D3 weighs twenty-five nanograms. That is a span of about 27,000× between two rows of the same table, and both are legitimately called “one unit”.
That last point has a concrete history. The older growth-hormone reference preparation, derived from pituitary extract, was assigned about 2.6 IU per mg. The recombinant standard that replaced it sits at 3 IU per mg. Run the same 10 IU vial through both: 10 ÷ 2.6 = 3.85 mg under the old factor, 10 ÷ 3 = 3.33 mg under the current one. A 15.4% difference in mass, from a vial whose label never changed. If you find an IU-per-mg figure in an old paper, check which standard it was written against before you use it.
Somatropin: about 3 IU per mg
Growth hormone is the compound most people are actually asking about when they search for an IU converter, and it is the easy one. The working factor:
1 mg somatropin = 3 IU, so 1 IU = 0.333 mg = 333 mcg
Divide IU by 3 to get milligrams. Multiply milligrams by 3 to get IU. That is the whole conversion. Common vial labels translate like this:
| Vial label | Mass of somatropin | Also written as |
|---|---|---|
| 4 IU | 1.33 mg | 1,333 mcg |
| 10 IU | 3.33 mg | 3,333 mcg |
| 12 IU | 4.00 mg | 4,000 mcg |
| 16 IU | 5.33 mg | 5,333 mcg |
| 36 IU | 12.00 mg | 12,000 mcg |
Notice the 12 IU and 36 IU rows land on clean milligram numbers, because 12 and 36 both divide by 3. The 10 IU vial — by far the most common presentation — does not, which is why 3.33 mg keeps showing up with a recurring decimal.
Worked example: a 10 IU vial in 1 mL
Three lines, in order, every time:
- Mass in the vial: 10 IU ÷ 3 IU/mg = 3.33 mg
- Concentration: 3.33 mg ÷ 1 mL = 3.33 mg/mL, which is also 10 IU/mL
- Per U-100 unit: 3.33 mg/mL × 0.01 mL = 33.3 mcg, or 0.1 IU per unit
So a 2 IU dose is 2 × 0.333 = 0.667 mg. Divide by the 3.33 mg/mL concentration: 0.667 ÷ 3.33 = 0.20 mL, which is 20 units. Cross-check it the lazy way — at 10 IU/mL, 2 IU is simply 2 ÷ 10 = 0.20 mL. Same answer, and that shortcut only works because the vial and the water happened to give a round IU/mL.
20 units on a U-100 syringe = 0.20 mL = 2 IU = 0.667 mg, from a 10 IU vial reconstituted with 1 mL.
The same vial at four reconstitution volumes
Adding more water does not change what is in the vial. It changes how far the dose spreads across the barrel, which is the only lever you have over precision.
| BAC water | mg/mL | IU/mL | IU per unit | 1 IU draw | 2 IU draw |
|---|---|---|---|---|---|
| 1 mL | 3.33 | 10 | 0.100 | 10 units (0.10 mL) | 20 units (0.20 mL) |
| 2 mL | 1.67 | 5 | 0.050 | 20 units (0.20 mL) | 40 units (0.40 mL) |
| 2.5 mL | 1.33 | 4 | 0.040 | 25 units (0.25 mL) | 50 units (0.50 mL) |
| 3 mL | 1.11 | 3.33 | 0.033 | 30 units (0.30 mL) | 60 units (0.60 mL) |
The 2.5 mL row is the tidiest of the four: 4 IU/mL means every whole IU lands on exactly 25 units and every quarter-IU on a countable 6.25. The 3 mL row pushes the recurring decimal onto the syringe, which is where you least want it. Picking the water volume so that your intended dose falls on a whole unit is the same trick covered in the general reconstitution chart.
hCG: about 9,000 to 10,000 IU per mg
Now run the identical procedure on a compound whose factor is three orders of magnitude away.
hCG is a much larger, heavily glycosylated molecule with high specific activity, and its reference standards land somewhere around 9,000–10,000 IU per mg. We are not going to publish a single midpoint as though it were exact — the band is the honest answer, and it is a band precisely because glycosylation varies. Where a number below needs one value, we use 9,500 IU/mg and say so.
A 5,000 IU vial therefore holds:
- At 10,000 IU/mg: 5,000 ÷ 10,000 = 0.50 mg
- At 9,500 IU/mg: 5,000 ÷ 9,500 = 0.53 mg
- At 9,000 IU/mg: 5,000 ÷ 9,000 = 0.56 mg
Roughly half a milligram. Half. A vial whose label reads five thousand contains less peptide by mass than a 2mg research vial of almost anything else.
| BAC water | IU/mL | IU per unit | 250 IU | 500 IU | 1,000 IU |
|---|---|---|---|---|---|
| 1 mL | 5,000 | 50 | 5 units | 10 units | 20 units |
| 2 mL | 2,500 | 25 | 10 units | 20 units | 40 units |
| 3 mL | 1,667 | 16.7 | 15 units | 30 units | 60 units |
| 5 mL | 1,000 | 10 | 25 units | 50 units | 100 units |
Take the 5 mL row and convert it to mass, using 9,500 IU/mg: 0.526 mg spread over 5 mL is 0.105 mg/mL, or 105 mcg/mL. Per U-100 unit that is 1.05 mcg. A 500 IU draw — 50 units, half a barrel — delivers about 52.6 mcg of peptide.
What happens if you transfer the factor anyway
Worth doing the arithmetic once, because the failure mode is not subtle and that is exactly what makes it easy to catch.
Apply somatropin’s 3 IU/mg to a 5,000 IU hCG vial and you conclude it contains 5,000 ÷ 3 = 1,667 mg — well over a gram and a half of peptide in a small glass vial. The true figure is around 0.53 mg, so the estimate is off by a factor of about 3,170.
Run it backwards. Apply hCG’s 9,500 IU/mg to a 10 IU somatropin vial and you get 10 ÷ 9,500 = 0.00105 mg, or 1.05 mcg, against a true 3.33 mg. Wrong by the same 3,170× in the other direction.
Somatropin at 3 IU per mg and hCG at 9,500 IU per mg, compared against a plain mg-labelled 5mg research vial. The IU number on a label tells you nothing about mass until you know the compound.
The reassuring part: errors this large are visible. A conversion that hands you 1.7 grams in a 3 mL vial, or a microgram where you expected milligrams, announces itself. Sanity-check the magnitude before you touch a syringe and the transfer error cannot survive.
What PeptiMap does and does not have on this
Plain statement of the data gaps, because they matter for what you can do next:
- No IU-labelled compound appears in our catalogue or our EU price index. There is no somatropin page, no hCG page, no vial size and no listed price for either. This article is arithmetic, not a product reference, and we are not going to link you to pages that do not exist.
- The IU-per-mg factors above come from reference-standard potency, not from our datasets. Everything in
peptides.tsand the price index is labelled in milligrams, so the site’s own numbers never needed an IU column. - The hCG figure is a range, not a value. Anywhere you see it quoted to four significant figures, someone has picked a midpoint and stopped mentioning it.
The mg-labelled GH-axis compounds we do carry sidestep the problem entirely — CJC-1295 5mg, ipamorelin 5mg and HGH Fragment 176-191 5mg are all measured in milligrams on the vial, so you skip straight to concentration.
Hand the milligrams to the calculator
The workflow is two steps and only the first one is IU-specific.
- Convert IU to mg using the factor for that compound. Divide by 3 for somatropin. Divide by 9,000–10,000 for hCG. Never reuse a factor across compounds.
- Convert mg to volume and units. The reconstitution calculator takes vial strength in mg, your BAC water volume and a target dose, and returns mg/mL, draw volume and the exact U-100 mark. It does not accept IU, which is why step one is yours.
If step two is the part that feels shaky, mcg and mg to insulin-syringe units walks the general case with no IU involved, and BAC water ratios covers how the water volume you pick changes the resolution of every draw afterwards.
Frequently Asked Questions
How many mg is 1 IU?
There is no answer without naming the substance. One IU of somatropin is about 333 mcg. One IU of hCG is about 0.105 mcg. One IU of vitamin E is 0.67 mg. The unit indexes biological activity against a per-substance reference standard, so the mass it corresponds to is different every time.
How do I convert IU to mg for growth hormone?
Divide by 3. Somatropin runs at roughly 3 IU per mg, so a 10 IU vial holds 3.33 mg, a 12 IU vial holds 4 mg and a 36 IU vial holds 12 mg. Going the other way, multiply milligrams by 3.
How many IU are in a mg of hCG?
Roughly 9,000 to 10,000, depending on the reference preparation. That puts a 5,000 IU vial at about 0.50 to 0.56 mg of peptide. We quote the band rather than a single figure because glycosylation genuinely varies between preparations.
How many units on the syringe is 2 IU of somatropin?
It depends on the concentration, not the IU. A 10 IU vial reconstituted with 1 mL gives 10 IU/mL, so 2 IU is 0.20 mL, or 20 units on a U-100 syringe. Reconstitute the same vial with 2 mL and 2 IU becomes 40 units. The vial label never sets the unit count — the water you added does.
Can I use the same IU-to-mg factor for two different peptides?
No, and the size of the error is the reason to care. Applying somatropin’s 3 IU/mg to a 5,000 IU hCG vial suggests 1,667 mg instead of about 0.53 mg — off by a factor of roughly 3,170. Look up the factor for the specific compound every time.
Does the reconstitution calculator accept IU?
Not directly. Convert to milligrams first, then enter the mg figure into the calculator along with your BAC water volume and target dose. It returns concentration in mg/mL, draw volume in mL and the U-100 unit mark.
Why do some vials use IU and others use mg?
History. Substances that were first produced by extraction from tissue — growth hormone, gonadotropins, insulin, heparin — had unreliable mass-to-activity ratios, so they were standardised by bioassay instead. Compounds that arrived later as defined synthetic peptides never needed that, which is why almost everything in a modern research catalogue is labelled in milligrams.
References
- World Health Organization Expert Committee on Biological Standardization. “WHO International Standards and Reference Reagents” — the ampouled reference preparations against which International Units are assigned by bioassay.
- Bristow AF, Jespersen AM. “The Second International Standard for Somatropin (Recombinant DNA-derived Human Growth Hormone): preparation and calibration in an international collaborative study.” Biologicals, 2001;29(2):97-106.
- Storring PL, Gaines Das RE. “The International Standard for Pituitary FSH and LH: collaborative study of the standard and of four other gonadotrophin preparations.” Journal of Endocrinology, 1989;123(2):275-293.
- Bangham DR, Mussett MV. “The Third International Standard for Insulin and the use of the International Unit for insulin potency.” Bulletin of the World Health Organization, 1959;20:1209-1220.
- Birken S, Berger P, Bidart JM, et al. “Preparation and characterization of new WHO reference reagents for human chorionic gonadotropin and metabolites.” Clinical Chemistry, 2003;49(1):144-154.
- PeptiMap. “Reconstitution Chart and Calculator” — the site’s mg-based concentration and syringe-unit tooling, which takes milligrams as its input and has no IU field.
This article is a research and educational reference only; it covers unit arithmetic rather than medical advice, and nothing described here is for human consumption.