How to Reconstitute a Peptide (With the Exact Math)

If you have ever stared at a vial of powder and a bottle of bacteriostatic water wondering what to do next, you are not alone. Reconstitution sounds technical, but it is just arithmetic. Here is the whole thing, plainly.

What “reconstitution” actually means

Your peptide arrives as a dry powder in a sealed vial. To use it, you add liquid (bacteriostatic water) to dissolve it. That is reconstitution. The amount of water you add sets the concentration, which is how much peptide sits in each millilitre of liquid.

The key thing to understand: the water does not change how much peptide you have. A 5 mg vial is 5 mg whether you add 1 mL or 3 mL. All the water changes is how concentrated the solution is, and therefore how much you draw for a given dose.

The only formula you need

Concentration = peptide amount ÷ water added.

Add 2 mL of bacteriostatic water to a 5 mg vial and you get 2.5 mg per mL. That is it. Everything else follows from this number.

From concentration to syringe units

Once you know your concentration, working out a dose is two more steps:

1. Volume to draw = your dose ÷ concentration. For a 250 mcg dose (that is 0.25 mg) at 2.5 mg/mL, you draw 0.1 mL.
2. Units on the syringe = volume × the syringe scale. A standard U-100 insulin syringe has 100 units per mL, so 0.1 mL reads as 10 units on the barrel.

That “units” number is the one that matters at the moment of injection, it is exactly where you pull the plunger to.

A worked example, start to finish

• Vial: 5 mg of peptide
• Water added: 2 mL → concentration is 2.5 mg/mL
• Target dose: 250 mcg
• Draw: 0.1 mL, which is 10 units on a U-100 syringe
• The vial holds 20 doses (5 mg ÷ 0.25 mg)

Rather not do the math by hand? The free Preptide calculator does all of this the moment you type in your numbers, and it has quick-fill presets for common compounds.

More water or less water?

This trips people up, so to be clear: adding more water makes the solution less concentrated, which means you draw more units for the same dose. Some people prefer that because larger volumes are easier to measure accurately on the syringe. Less water does the opposite. Neither changes the dose itself, only how many tick marks it lands on.

A note on safety

This is arithmetic, not medical advice. How much of any compound to take, and whether to take it at all, is a decision for you and a licensed provider. This guide only shows you how the reconstitution math works so the numbers are not a mystery. Always double-check your figures before drawing a dose.