Mass units convert by fixed factors: 1 g = 1,000 mg, and 1 mg = 1,000 mcg. International Units (IU) do not. An IU measures biological activity against a WHO reference standard for one specific substance, so an IU-to-mg factor works only for that substance. For somatropin, the current WHO standard sets 1 mg = 3.0 IU. Our peptide unit converter handles the arithmetic.
Notice: Research peptides are sold for in-vitro laboratory research only. They are not FDA-approved, not for human or animal consumption, and this page is not medical advice.
This page covers unit arithmetic. It doesn't suggest amounts of anything.
IU to mg calculator: the factor you need
To convert IU to mg, you need the factor for that exact substance: mg = IU ÷ (IU per mg). Enter that factor in our free IU to mg calculator and it does the rest. These are the official factors for the substances people convert most often:
| Substance | Factor | Example | Source |
|---|---|---|---|
| Vitamin D (D2 or D3) | 1 mcg = 40 IU | 1,000 IU = 25 mcg; 50 mcg = 2,000 IU | NIH ODS |
| Vitamin E, natural (d-alpha-tocopherol) | 1 IU = 0.67 mg | 400 IU = 268 mg | NIH ODS |
| Vitamin E, synthetic (dl-alpha-tocopherol) | 1 IU = 0.45 mg | 400 IU = 180 mg | NIH ODS |
| Vitamin A, retinol | 1 IU = 0.3 mcg RAE | 3,000 IU = 900 mcg RAE | NIH ODS |
| Somatropin (current WHO standard) | 1 mg = 3.0 IU | 10 IU = 3.33 mg | NIBSC 98/574 |
mg = IU ÷ (IU per mg)
mcg = IU ÷ (IU per mcg)
IU = mg × (IU per mg)
Vitamin D: IU to mcg and mg table (1 mcg = 40 IU)
| IU | mcg | mg |
|---|---|---|
| 100 IU | 2.5 mcg | 0.0025 mg |
| 400 IU | 10 mcg | 0.01 mg |
| 1,000 IU | 25 mcg | 0.025 mg |
| 1,600 IU | 40 mcg | 0.04 mg |
| 2,000 IU | 50 mcg | 0.05 mg |
| 3,200 IU | 80 mcg | 0.08 mg |
| 5,000 IU | 125 mcg | 0.125 mg |
| 10,000 IU | 250 mcg | 0.25 mg |
| 20,000 IU | 500 mcg | 0.5 mg |
| 50,000 IU | 1,250 mcg | 1.25 mg |
Read it either way: 500 micrograms of vitamin D is 20,000 IU, and 2,000 IU is 50 mcg (0.05 mg). These values apply to vitamin D only; NIH ODS is the source of the factor. For any other substance, use its own factor in the IU to mg calculator.
If your substance isn't in a table like this one, there is no safe guess: the factor comes from its own reference standard, and most research peptides have none. The rest of this page explains why.
Mass units: g, mg and mcg
Most research peptides are labeled by mass, usually in milligrams (mg). Research papers and calculations often use micrograms (mcg, sometimes written µg). The metric prefixes are fixed:
- 1 gram (g) = 1,000 milligrams (mg)
- 1 milligram (mg) = 1,000 micrograms (mcg)
- 1 gram (g) = 1,000,000 micrograms (mcg)
To convert:
mg → mcg: multiply by 1,000
mcg → mg: divide by 1,000
g → mg: multiply by 1,000
mg → g: divide by 1,000
Mass conversion table
| Milligrams (mg) | Micrograms (mcg) | Grams (g) |
|---|---|---|
| 0.1 mg | 100 mcg | 0.0001 g |
| 0.25 mg | 250 mcg | 0.00025 g |
| 0.5 mg | 500 mcg | 0.0005 g |
| 1 mg | 1,000 mcg | 0.001 g |
| 2 mg | 2,000 mcg | 0.002 g |
| 5 mg | 5,000 mcg | 0.005 g |
| 10 mg | 10,000 mcg | 0.01 g |
| 1,000 mg | 1,000,000 mcg | 1 g |
The thousand-fold trap
The most common error in peptide arithmetic is reading mg as mcg or the reverse. The mistake is exactly 1,000 times. Vial labels are usually in mg, and amounts in research papers are often in mcg, which is where the mix-up starts. Before any calculation, write the unit next to every number.
Concentration units
Once a peptide is dissolved, you work with concentration, which is mass per volume:
concentration (mg/mL) = vial mass (mg) ÷ diluent volume (mL)
concentration (mcg/mL) = concentration (mg/mL) × 1,000
mcg per 0.1 mL = concentration (mcg/mL) × 0.1
Example: a 5 mg vial with 2 mL of diluent is 2.5 mg/mL, which is 2,500 mcg/mL, or 250 mcg in every 0.1 mL. These are the same formulas our reconstitution calculator uses. See how to reconstitute peptides for the full procedure.
Label mass vs peptide mass
The mg on a label may include more than the peptide itself. Bachem notes that "the peptide content of the solid may range from 70 to 90% as peptides contain counter ions (e.g. acetate, trifluoroacetate) and residual moisture" (Bachem). If a COA reports net peptide content, that figure is the more precise input. See how to read a peptide COA.
International Units: why IU is substance-specific
An International Unit isn't a unit of mass or volume. It is a unit of biological activity, defined by comparison with a physical reference material.
The World Health Organization maintains International Biological Reference Standards "to define an internationally agreed unit to allow comparison of biological measurements worldwide," and says that where appropriate "a value is assigned to the WHO IBRS (for example, a potency value in International Units)" (WHO). The UK's NIBSC, which holds many of these standards, explains why: biological medicines' activity can't be quantified "in System Internationale (SI) units (e.g. mass), so they must be measured using biological or immunological assays" (NIBSC).
Three consequences follow:
- 1 IU of one substance is unrelated to 1 IU of another. The IU for somatropin and the IU for insulin are defined by different standards.
- There's no universal IU-to-mg factor. You need the factor for that exact substance.
- The factor can change. When WHO replaces a standard, the assigned value can shift.
Worked example: somatropin (1 mg = 3 IU)
Somatropin, recombinant human growth hormone, is the best-known peptide product labeled in IU.
The WHO's second International Standard for somatropin, NIBSC code 98/574, contains 1.95 mg of somatropin per ampoule and is assigned "3.0 International units per mg Somatropin" (NIBSC instructions for use). The standard's calibration study is published in Biologicals (2001).
Approved product labeling reflects this. The European label for Omnitrope states that each mL of the 5 mg/1.5 mL product "contains 3.3 mg of somatropin (corresponding to 10 IU)" (EMA), which is about 3 IU per mg.
Older documents use a different factor. A 2006 FDA-approved Humatrope label footnotes a figure "based on previous International Standard of 2.7 IU=1 mg" (FDA label). If you're comparing sources, check which standard each one uses.
Somatropin conversion table (3.0 IU per mg)
| Somatropin (mg) | Somatropin (mcg) | IU (3.0 IU/mg) |
|---|---|---|
| 0.1 mg | 100 mcg | 0.3 IU |
| 0.333 mg | 333 mcg | 1 IU |
| 1 mg | 1,000 mcg | 3 IU |
| 1.667 mg | 1,667 mcg | 5 IU |
| 3.333 mg | 3,333 mcg | 10 IU |
| 5 mg | 5,000 mcg | 15 IU |
| 10 mg | 10,000 mcg | 30 IU |
IU = mg × 3.0 (somatropin only)
mg = IU ÷ 3.0 (somatropin only)
Don't apply this factor to anything else. It is specific to somatropin and its WHO standard. For research peptides labeled only in mg, stay in mg and mcg.
Syringe "units" are not International Units
The word "unit" means two unrelated things:
| Term | What it measures | Example |
|---|---|---|
| International Unit (IU) | Biological activity vs a WHO standard | 3.0 IU per mg of somatropin |
| Syringe unit (U-100) | Volume: 0.01 mL per unit | 10 units = 0.1 mL |
| Syringe unit (U-40) | Volume: 0.025 mL per unit | 4 units = 0.1 mL |
They only match for insulin drawn with the syringe scale made for it. For peptides labeled in mg, a syringe unit is just a volume mark. See U-40 vs U-100 syringes.
A four-step conversion checklist
- Write the unit next to every number. mg, mcg, IU, mL or syringe units.
- Convert mass first. Get everything into mcg or everything into mg.
- Use IU only with a named, substance-specific factor, and cite the standard it comes from.
- Check the answer's size. If a volume is larger than the diluent you added, a unit is wrong.
Use the peptide unit converter for mass and IU conversions, and the reconstitution calculator for concentration and syringe units. The research peptide glossary defines the other terms you'll see on labels.
Unit math depends on an honest label
Every conversion here assumes the vial holds what it says. Check that a vendor publishes lot-matched third-party testing before trusting its labels. Our ranking of the best place to buy peptides online scores US vendors on testing, COAs and labeling. It is an editors' ranking, not a sort by score (how we rank).
Sources are linked inline. Last reviewed October 6, 2026.