Peptide calculator: from vial to syringe units

Every result on this site comes from the same three steps. First the concentration: the amount of peptide in the vial divided by the water added. Then the volume: your dose divided by the concentration. Last the units: on a U-100 insulin syringe, 1 mL is 100 units, so the volume in mL times 100 is the line to draw to.

Doses in micrograms are converted once, at the start: 1 mg = 1,000 mcg. After that everything is worked in mg, which is where most mix-ups are caught.

Standard U-100 insulin syringes hold 100 units in 1 mL, so each unit is 0.01 mL. They come in 0.3 mL (30 units), 0.5 mL (50 units) and 1 mL (100 units) sizes. A unit is the same amount of liquid on all three; the smaller barrels are simply easier to read for small volumes.

MedlinePlus describes each small notch on a standard 1 mL insulin syringe as 1 unit, and some 0.3 mL barrels are also marked in half units. Markings differ by brand, so look at your own barrel before relying on a result that falls between two lines, and ask your pharmacist to show you how to read it.

A syringe unit is a volume. How much peptide it holds depends entirely on how strong the vial is. Ten units from a strong mix and ten units from a weak mix are different doses, which is why the calculator always starts from mg and never from units alone.

If you change how much water goes into a vial, the units for the same dose change too. Keep the concentration written on the vial and re-check the units whenever a new vial is mixed differently.

It refuses to show a result when the dose will not fit in the chosen syringe or is larger than the whole vial. It warns when a result is under 2 units (a threshold we chose because such small volumes are hard to read on the lines), or so small or large that mg and mcg were probably mixed up. It also warns when you pick a U-40 syringe: U-40 veterinary insulin has 40 units per mL, and its label warns that any other syringe gives the wrong dose.