IU to mg
An international unit measures biological activity against a reference standard, not mass. There is no universal factor, so no converter can be honest here.
Run the math in IU instead
If a vial is labeled in IU, keep IU as the mass unit throughout. The three steps do not care what the unit is called:
IU per draw ÷ IU/mL = mL
mL × 100 = U-100 units
The main calculator accepts the numbers unchanged: enter the IU figure in the vial field and the IU per draw in the amount field, and read the unit labels as IU.
What an IU is
An international unit is a quantity defined by an agreed reference preparation for one specific substance. The mass that corresponds to one IU is whatever mass of that substance produces the reference activity, and it is different for every substance that uses the unit. Two materials labeled 10 IU can differ in mass by orders of magnitude. For that reason a general IU-to-mg converter cannot exist, and any page offering one is either silently assuming one substance or is wrong.
Where mg is the honest unit
Synthetic peptides are made to a mass and the lot certificate reports mass: identity by mass spectrometry, purity by HPLC, sometimes a net peptide content. Labels for these materials read in milligrams, and all of the arithmetic on this site starts from that milligram figure. If a listing quotes a peptide in IU with no reference standard named, the number cannot be checked against the certificate.
If you have a factor
When a substance's reference standard is published, the conversion is a multiplication: IU × (mg per IU) = mg. Do that once, write the mg on the vial, and the rest of the math is ordinary. This site does not publish factors for any substance, because it does not describe the substances themselves, only the arithmetic.
Questions
How many mg is 1 IU?
There is no single answer. An international unit measures activity, not mass, and each substance that uses IU has its own factor set by a reference standard. Without the substance and its factor the question cannot be answered.
Why do some vials say IU and others mg?
Materials standardized by biological activity are labeled in IU; synthetic peptides, whose mass is what the lot certificate measures, are labeled in mg. Most research peptide vials are in mg.
Can I use the reconstitution calculator with IU?
Only after converting the vial content to a mass with the factor for that specific substance, or by treating IU as the mass unit throughout: IU in the vial divided by mL gives IU per mL, and IU per draw divided by that gives the volume. The arithmetic is the same.