Reconstitution math, step by step

How to convert a vial of dry powder and a volume of bacteriostatic water into a working concentration in mg per mL, and how to use that concentration to calculate the syringe volume for a target dose.

A vial of dry powder plus a vial of bacteriostatic water plus a syringe equals a dose only if you can do the arithmetic. This page walks through the calculation in plain steps, with worked examples, and shows where the common mistakes happen. The goal is to make the math obvious, not to recommend any specific compound, dose or protocol — those are clinical decisions, and they belong with the prescriber who supplied the medication.

If your medication came with a manufacturer pre-mixed presentation (an oil vial already at a stated mg/mL, or a pre-filled pen), no reconstitution is needed — read the label and skip to the syringe-volume section.

The two numbers you need

Every reconstitution boils down to two numbers:

Total active mass in the vial. This is on the label, in milligrams (peptides usually) or international units (some hormones). Examples: a 5 mg vial of BPC-157, a 10 mg vial of semaglutide, a 12 mg cartridge of growth hormone.
Volume of bacteriostatic water (BAC) you add. This is your choice within practical limits. The label rarely tells you exactly what volume to use; the manufacturer gives a range or leaves it to the prescriber.

The resulting concentration in mg/mL is:

concentration (mg/mL) = total mass (mg) / volume added (mL)

That single formula is the whole game.

A worked example with whole numbers

Suppose you have a 5 mg vial of a peptide and you add 1 mL of bacteriostatic water.

5 mg / 1 mL = 5 mg/mL

So every 1 mL of the reconstituted solution contains 5 mg of active compound. Half a millilitre contains 2.5 mg. A quarter of a millilitre contains 1.25 mg.

If you instead add 2 mL of bacteriostatic water:

5 mg / 2 mL = 2.5 mg/mL

Now every 1 mL contains only 2.5 mg, and half a millilitre contains 1.25 mg. Same total dose available in the vial — just diluted twice as much, so each syringe pull contains half as much active drug.

The choice of dilution volume is yours, within practical limits. More water means each dose is a larger and easier-to-measure volume. Less water means each dose is smaller — convenient for one or two doses, harder to measure precisely.

Converting to syringe volume for a target dose

Once you know the concentration, the syringe volume for a target dose is:

syringe volume (mL) = target dose (mg) / concentration (mg/mL)

Worked example: you want a 250 µg dose (i.e. 0.25 mg) from a 5 mg/mL solution.

0.25 mg / 5 mg/mL = 0.05 mL

That is 5 units on a U-100 insulin syringe (since 1 mL = 100 units on that scale). See the U-100 syringes and dose precision article for the unit conversion in more detail.

A worked example with growth hormone IUs

Growth hormone is labelled in international units (IU). The standard conversion is 3 IU ≈ 1 mg of recombinant human GH (manufacturer-specific; check the label, because the conversion can vary slightly by brand).

A 12 IU cartridge therefore contains roughly 4 mg of active GH. If you reconstitute that into 1 mL of bacteriostatic water:

12 IU / 1 mL = 12 IU/mL

A 2 IU dose is then 2 / 12 = 0.167 mL, or about 17 units on a U-100 syringe. Many people prefer to reconstitute GH at a fixed IU/mL that makes the syringe pull a whole number of units — for example, adding 1 mL to a 10 IU pen gives 10 IU/mL, and a 1 IU dose is then exactly 10 units. This is purely a convenience choice; it does not change the total active mass in the vial.

How to do it once, then refer back

In DoseCurve, when you mark a vial as opened on the timeline, the reconstitution panel records:

The label mass of the vial (mg).
The volume of BAC water you added (mL).
The derived concentration (auto-calculated as mg / mL).
A free-text notes field for anything else you want to remember (BAC water batch, peptide brand, reconstitution date, anything).

The reconstitution timeline above the chart then displays each vial event with all of those fields, so you can scroll back weeks or months later and see exactly how the vial currently in use was reconstituted. That matters because the syringe volume for any given dose depends on the concentration — and getting that wrong is the single most common reconstitution-related dosing error.

Common mistakes, and how to avoid them

Reading the mass per syringe instead of per mL. The concentration is per mL. If your syringe pull is 0.5 mL of a 5 mg/mL solution, the dose is 2.5 mg, not 5 mg.
Mixing mg and µg in the same calculation. Convert everything to one unit before you start. 1 mg = 1000 µg. A "100 µg" dose from a 5 mg/mL solution is 0.1 / 5 = 0.02 mL, which is 2 units on a U-100 syringe.
Forgetting BAC water is not perfectly displaced. When you add 1 mL of water to a vial containing 5 mg of dry powder, the total liquid volume is essentially 1 mL (the powder displaces a negligible volume at these masses). You do not need to subtract anything.
Reconstituting too dilute for tiny doses. A 5 mg vial in 5 mL of water gives 1 mg/mL — convenient if your dose is 1 mg, but a 100 µg dose is then 0.1 mL = 10 units. If your usual dose is 50 µg, you'll be pulling 5 units, and the precision of a U-100 syringe at 5 units is poor. Concentrate more.
Reconstituting too concentrated for large doses. A 5 mg vial in 0.5 mL gives 10 mg/mL — convenient if doses are small, but a 2 mg dose is then 0.2 mL, which is past the comfortable measurement range of a small insulin syringe.
Not recording the dilution. A vial opened two weeks ago at an unknown dilution becomes a guessing game. Use the reconstitution timeline.

Stability after reconstitution

Most peptide manufacturers specify a refrigerated stability of 28 days after reconstitution with bacteriostatic water (which contains 0.9% benzyl alcohol as a preservative). Plain sterile water without preservative typically halves that. The exact figure depends on the peptide and the manufacturer — check the product insert.

DoseCurve's vial timeline lets you record the open date so you can see at a glance how long the current vial has been in use. The "replace" event marks the start of a new vial and its own reconstitution record.