Reconstitution calculator.

How the math works

Reconstitution is two divisions and a multiplication, which is exactly why it's easy to get quietly wrong at 6am.

Adding bacteriostatic water to a lyophilised vial gives you a concentration: the vial's strength divided by the volume of water. A 10 mg vial with 2 mL of water is 5 mg/mL. The water volume does not change how much peptide you have, only how concentrated it is.

To get a dose, divide it by that concentration. A 250 mcg dose (0.25 mg) at 5 mg/mL is 0.05 mL of liquid.

Insulin syringes aren't marked in millilitres, they're marked in units: 100 graduations per mL on a U-100. So 0.05 mL is 5 units. That last step is the one people skip, and it's a factor-of-100 error when they do.

"Units" here means syringe graduations, not international units. They are unrelated measures that share a name, and conflating them is its own category of mistake.

Getting it wrong in the safe direction

More water is not more drug, it just spreads the same peptide across a bigger volume, which means more units drawn for the same dose. That's usually the safer way to be imprecise, because small errors on a longer draw matter proportionally less.

If the calculator says your draw is more than the barrel holds, it will tell you. That's the failure worth catching: it means the numbers don't fit the syringe you picked, not that you should draw twice.