Reconstitution and syringe-unit calculator

The most consequential error in this field is arithmetic. Not technique, not timing, not storage — arithmetic. A person who has understood every word of the pharmacology can still put a wrong number on a syringe, and the reason is that three different units are in play at once: milligrams of peptide, millilitres of solution, and the graduation marks on an insulin syringe, which are neither.

This page does the conversion. It is a calculator, not advice: it tells you what a given set of numbers implies, and it has no opinion about whether those numbers are the right ones. Nothing you type is transmitted anywhere; the whole thing runs in the page.

How the arithmetic works

Three steps, and each one is a single division.

1. Concentration. Vial mass divided by diluent volume. A 5 mg vial reconstituted with 2 mL is 2.5 mg per mL.
2. Volume per dose. Intended dose divided by concentration. A 0.5 mg dose from a 2.5 mg/mL solution is 0.2 mL.
3. Units on the syringe. Volume multiplied by 100, because every U-100 insulin syringe has 100 graduation marks per millilitre. 0.2 mL is 20 units.

The third step is where the errors cluster, because the word "unit" on an insulin syringe is a volume marking that exists for a completely unrelated reason: it was designed so that one graduation delivers one international unit of U-100 insulin. It has nothing to do with the peptide in the vial. A "unit" is 0.01 mL, and that is all it is.¹

Why the content correction field is there

A vial labelled 5 mg does not, in the general case, contain 5 mg of peptide. Lyophilised peptides are supplied as salts — usually trifluoroacetate or acetate — and the counter-ion contributes mass, as does residual water in a hygroscopic powder and residual solvent that lyophilisation did not fully remove. A vial can be 99% pure by chromatography and 84% peptide by mass, and both figures can be correct.

Across nine vials the Journal submitted for nitrogen determination in 2024, the median was 93% of labelled mass and the lowest was 81%. If you have a content figure for your material, entering it here shows you what the shortfall does to every number downstream. If you do not have one, the calculator assumes the label is the peptide mass, which is the assumption most people are unknowingly making.

The five errors we see most often

1. Treating units as milligrams. "I take 20 units" is a statement about volume and says nothing about dose until the concentration is known. Two people both taking 20 units can be an order of magnitude apart.
2. Reconstituting to a different volume than planned and not recalculating. If the intention was 2 mL and 1 mL went in, every subsequent dose is doubled.
3. Confusing milligrams and micrograms. A factor of a thousand. This is why the calculator makes you choose the unit rather than guessing.
4. Assuming the label mass is peptide mass. Discussed above; typically a 4–19% error in the same direction.
5. Using a non-U-100 syringe on U-100 arithmetic. U-40 and U-500 syringes exist and their graduations are not 0.01 mL. Check the barrel.

What this page does not tell you

It does not tell you what dose to take, whether a dose is appropriate, whether a product is what it claims to be, or whether any of this should be happening at all. It converts numbers. The Journal reports on this field; it does not practise in it, and a calculator is not a clinician.

Worked example, in full

A vial labelled 10 mg. Reconstituted with 2.0 mL of bacteriostatic water. Concentration is 10 ÷ 2.0 = 5.0 mg/mL. An intended dose of 2.5 mg requires 2.5 ÷ 5.0 = 0.50 mL, which on a U-100 syringe is 50 units — the entire barrel of a 0.5 mL syringe. The vial contains 10 ÷ 2.5 = 4 such doses.

Now suppose a content determination returns 90% of label. The vial holds 9.0 mg, the true concentration is 4.5 mg/mL, and 50 units delivers 2.25 mg rather than 2.5 mg — a 10% shortfall, invisible at every point in the process. To deliver 2.5 mg you would need 0.556 mL, or 55.6 units, which a 0.5 mL syringe cannot hold. That is the practical consequence of a number nobody printed on the certificate.