Peptide units explained: mg, mcg, IU and the numbers on a syringe

Four numbering systems, one bench

A lyophilised peptide arrives labelled in milligrams. The literature describing it reports quantities in micrograms. Some related compounds are specified in international units. And the syringe used to measure it is marked in something called “units” that is none of the above.

Four systems, describing three genuinely different things — mass, biological activity, and volume — and only one pair of them converts by arithmetic alone. Most of the errors people make here come from treating all four as interchangeable scales of the same quantity. They are not.

1. Mass: milligrams and micrograms

This is the straightforward part.

• 1 gram (g) = 1,000 milligrams (mg)
• 1 milligram (mg) = 1,000 micrograms (mcg)

mcg and µg are the same unit. The symbol is the Greek letter mu; “mcg” is the same thing written without it, a convention that exists because µ is easy to mistake for an m in handwriting and easy to lose in plain-text systems. If a source writes 250 µg and another writes 250 mcg, they mean the identical quantity.

Vials are labelled in milligrams because that is a convenient size for the contents — 5 mg, 10 mg. Research literature reports in micrograms because the quantities under study are usually a small fraction of a vial. Both describe mass, so moving between them is a decimal shift and nothing more:

• 10 mg = 10,000 mcg
• 0.25 mg = 250 mcg
• 500 mcg = 0.5 mg

2. IU is not a mass

An international unit is a measure of biological activity, not of quantity. It exists for substances where what matters is the effect produced rather than the weight of material, and where preparations of differing purity would otherwise not be comparable.

The critical consequence: the IU is defined separately for every substance it applies to. There is no universal conversion, and an IU of one thing has no fixed relationship to an IU of another. Each definition is fixed by an international reference preparation held for that purpose.

Most research peptides have no IU definition at all. They are specified by mass, because there is no reference preparation defining activity units for them. If you find yourself trying to convert a peptide quantity into IU, the usual explanation is that the source was describing a different compound.

3. The numbers on a syringe are a volume

This is the one that costs people most, and it is worth being blunt about.

An insulin syringe is marked in “units” from 0 to 100. Those markings measure volume. On a standard U-100 syringe:

• 100 units = 1 mL
• 1 unit = 0.01 mL = 10 microlitres

That is the whole definition. A unit is a hundredth of a millilitre, and it stays a hundredth of a millilitre regardless of what the syringe contains — water, a peptide solution at 1 mg/mL, or the same peptide at 10 mg/mL.

The reason the markings say “units” rather than “0.01 mL” is historical. The syringe was designed for insulin, which is supplied at a standardised concentration of 100 IU per mL — which is what U-100 means. At that specific concentration, and only at that concentration, one unit of volume happens to contain exactly one international unit of insulin, so the two numbers coincide and the barrel can be marked with either.

For anything that is not insulin at 100 IU/mL, that coincidence does not hold. The number on the barrel tells you how much liquid you have drawn and nothing whatsoever about how much substance is in it. How much substance is in it depends entirely on the concentration you prepared, which is the next section.

Syringes also come in different barrel sizes — 0.3 mL, 0.5 mL and 1.0 mL are common. All are marked in the same 0.01 mL units; the smaller barrels simply stop sooner (at 30 and 50 units respectively) and spread those markings further apart, which makes small volumes easier to read accurately.

4. The one equation

Once the units are straight, the arithmetic is a single relationship used twice.

Preparing the solution — the concentration is the mass in the vial divided by the volume of solvent added to it:

Measuring from it — the volume containing a target mass is that mass divided by the concentration:

Note what determines the answer: the mass in the vial and the volume of solvent. Adding more solvent does not change how much peptide exists — it spreads the same mass through more liquid, so each measured volume contains proportionally less. That is the entire reason a concentration has to be calculated rather than assumed.

5. Worked example

Change one input and follow it through. Reconstitute the same 10 mg vial with 1 mL instead of 2 mL and the concentration doubles to 10 mg/mL, so the same 250 mcg now occupies 0.025 mL — two and a half units, a mark that is genuinely hard to read on a 1 mL barrel. Using more solvent buys measuring accuracy at small quantities, which is usually the reason to do it.

6. Where this goes wrong

• Reading syringe units as micrograms. The single most common error, and the most expensive: someone reads “250” in a protocol, draws to a number, and has measured a volume that bears no relationship to it. On the vial above, 250 mcg is five units, and there is no syringe on which it would be 250.
• Assuming the label is the content. A vial labelled 10 mg is specified at 10 mg; what it actually contains is whatever the certificate of analysis measured. Ours for BPC-157 measured 11.44 mg against a 10 mg label — 14% more than the label says, which propagates straight into every concentration calculated from it. How to read a certificate of analysis covers where to find that figure.
• Confusing purity with content. A separate trap in the same family: a peptide at 99% chromatographic purity is not 99% peptide by weight. Same article, last section.
• Mixing units mid-calculation. Convert everything to one system before dividing. Milligrams throughout, or micrograms throughout — not one of each.
• Assuming an IU conversion exists. Covered above; it is substance-specific or it does not exist.

Check the arithmetic

Our peptide reconstitution calculator does the two calculations above: enter the vial quantity, the solvent volume and the syringe size, and it returns the concentration and the volume for a target quantity. It is worth doing one calculation by hand first, though — a calculator that returns a number you cannot sanity-check is a way of being wrong faster.

Terms used here are defined in the peptide glossary.

For laboratory research use only. Not for human consumption. The materials referenced on this page are not drugs, foods, or cosmetics and may not be sold or used for any purpose other than in vitro or non-human laboratory research.