Reconstitution math: turning mg per mL into insulin syringe units

Reconstituting a peptide vial is a two step math problem, and most dosing mistakes happen in the second step.

Step one is concentration. A 5 mg vial with 2 mL of bacteriostatic water gives 2.5 mg/mL. Same vial with 1 mL gives 5 mg/mL. The powder amount never changes, only the water you add, so the vial label alone tells you nothing about what one unit of your syringe holds.

Step two is the syringe. Insulin syringes are marked in units, not millilitres. A U-100 syringe has 100 units per 1 mL, so one unit is 0.01 mL. At 2.5 mg/mL that unit carries 0.025 mg, or 25 mcg. A 250 mcg dose is therefore 10 units. At 5 mg/mL the same 250 mcg dose is 5 units. Halving your diluent halves the mark you draw to.

The formula that covers every case:

units = (dose in mg / concentration in mg/mL) * 100 Three habits that have saved me from bad draws:

1. Write the reconstitution date and the mL of water directly on the vial cap. A vial with no note is a vial you should not dose from.
2. Recompute units whenever you open a new vial, even of the same product. Batch sizes change.
3. Keep a dose log with date, site, dose in mg and units drawn. When something feels off, the log tells you whether it was a dosing error or the compound.

I keep the arithmetic and the log in one place with a Peptide Tracker & Calculator rather than a spreadsheet, mostly because the unit conversion is where I used to slip. Whatever you use, the important part is that the number of units is derived from the concentration you actually mixed, not from a number you read on a forum.