How the calculation works

concentration (mcg/mL) = vial strength (mg) × 1000 ÷ water added (mL)

Everything follows from concentration. The vial contains a fixed mass of peptide; adding bacteriostatic water changes only the volume it is dissolved in, never the amount. A 5 mg vial reconstituted with 2 mL gives 2,500 mcg/mL. The same vial with 1 mL gives 5,000 mcg/mL — twice the concentration, the same total drug.

To convert a target dose into syringe units, the calculator divides the dose by the concentration to get a volume, then multiplies by the units-per-mL of your syringe. A U-100 insulin syringe is marked so that 100 units equal 1 mL, so each unit is 0.01 mL.

A worked example

5 mg vial + 2 mL water → 2,500 mcg/mL → 250 mcg dose = 0.1 mL = 10 units (U-100)

Take a 5 mg vial of BPC-157 and add 2 mL of bacteriostatic water. Concentration is 5 × 1000 ÷ 2 = 2,500 mcg/mL. For a 250 mcg dose you need 250 ÷ 2,500 = 0.1 mL. On a U-100 syringe that is 10 units — the first major graduation, which is convenient and easy to read.

The vial holds 5,000 mcg total, so at 250 mcg per dose it contains 20 doses. Change nothing except the water volume — 1 mL instead of 2 mL — and the same 250 mcg dose becomes 5 units. The dose is identical; only the mark you draw to has moved.

This is why 'how many units should I inject' is an unanswerable question without knowing the reconstitution. Units are a volume measurement, not a dose.

Choosing a water volume

More water makes each dose a larger volume, which lands further along the syringe barrel and is easier to measure accurately. If your calculated dose comes out below about 5 units, consider reconstituting with more water so the measurement error matters less — a one-unit misread on a 4-unit dose is a 25% error.

The counter-pressure is capacity and stability. Insulin syringes hold 0.3–1 mL, so very dilute solutions may not fit in a single draw, and reconstituted peptides have a limited refrigerated shelf life. Bacteriostatic water contains 0.9% benzyl alcohol as a preservative, which is what makes multi-dose use over weeks possible; sterile water without a preservative is single-use.

Frequently asked questions

How do I calculate peptide reconstitution?

Divide the vial strength in milligrams by the volume of bacteriostatic water in millilitres, then multiply by 1,000 to get micrograms per millilitre. Divide your target dose in micrograms by that concentration to get the injection volume in millilitres. Multiply by 100 for units on a U-100 insulin syringe. A 5 mg vial in 2 mL gives 2,500 mcg/mL, so a 250 mcg dose is 0.1 mL, or 10 units.

Does adding more bacteriostatic water make the peptide weaker?

It makes the solution less concentrated but does not change the amount of peptide you inject. The vial contains a fixed mass. Adding more water means each dose is a larger volume containing the same quantity of drug. Doubling the water halves the concentration and doubles the number of syringe units for the same dose.

How many units is 250 mcg of BPC-157?

It depends entirely on how the vial was reconstituted. A 5 mg vial in 2 mL of bacteriostatic water gives 2,500 mcg/mL, making 250 mcg equal to 10 units on a U-100 syringe. The same vial in 1 mL gives 5,000 mcg/mL, making 250 mcg equal to 5 units. Units measure volume, not dose.

What is the difference between bacteriostatic and sterile water?

Bacteriostatic water contains 0.9% benzyl alcohol, which inhibits bacterial growth and allows a vial to be entered multiple times over a period of weeks when refrigerated. Sterile water has no preservative and is intended for single use. For multi-dose peptide vials, bacteriostatic water is the standard choice.

Why does my calculated dose come out at less than 5 units?

Because the solution is quite concentrated relative to your dose. Small volumes are harder to measure precisely — misreading by one unit on a 4-unit draw is a 25% dosing error. Reconstituting the next vial with more water spreads the same dose across more syringe units and reduces the impact of measurement error.