How to Reconstitute a Peptide Vial

The answer in one line

Units to draw = (Target dose mcg ÷ Total peptide mcg) × Bac water mL × 100. For a 5 mg vial, 2 mL bac water, 250 mcg dose: (250 ÷ 5000) × 2 × 100 = 10 units on a U100 syringe.

If you just need the number right now, use the calculator and skip the rest of this page. If you want to understand why, keep reading.

What reconstitution actually is

Most injectable peptides are sold as lyophilized (freeze-dried) powder. Freeze-drying removes moisture for stability during shipping and storage. Before you can inject, you dissolve the powder in bacteriostatic water (bac water) — a sterile water solution with 0.9% benzyl alcohol that inhibits bacterial growth.

The result is a solution with a known concentration: total peptide divided by total bac water volume. Every dose you draw is a fraction of that solution.

The formula, step by step

Three inputs. One output.

1. Vial strength (mcg) — the total amount of peptide in the vial. Vials are labeled in mg; convert to mcg by multiplying by 1,000. (5 mg = 5,000 mcg. 10 mg = 10,000 mcg.)
2. Bac water added (mL) — how much bacteriostatic water you inject into the vial. You choose this number. Common amounts are 1 mL, 2 mL, or 3 mL.
3. Target dose (mcg) — the amount you want to inject per dose. This comes from your prescribing provider.

The math:

• Concentration = Vial strength (mcg) ÷ Bac water (mL)
• Volume to draw (mL) = Target dose (mcg) ÷ Concentration (mcg/mL)
• Units on U100 syringe = Volume (mL) × 100

Or collapsed into one step: Units = (Target dose ÷ Vial strength) × Bac water × 100.

Three worked examples

Example 1 — 5 mg vial, 2 mL bac water, 250 mcg dose

Concentration: 5,000 mcg ÷ 2 mL = 2,500 mcg/mL. Volume: 250 mcg ÷ 2,500 mcg/mL = 0.10 mL. Units: 0.10 × 100 = 10 units.

Example 2 — 5 mg vial, 1 mL bac water, 250 mcg dose

Concentration: 5,000 mcg ÷ 1 mL = 5,000 mcg/mL. Volume: 250 mcg ÷ 5,000 mcg/mL = 0.05 mL. Units: 0.05 × 100 = 5 units.

Same vial, same dose, half the units — because you used half the bac water. The concentration doubled. This is the most common source of dosing errors.

Example 3 — 10 mg vial, 2 mL bac water, 500 mcg dose

Concentration: 10,000 mcg ÷ 2 mL = 5,000 mcg/mL. Volume: 500 mcg ÷ 5,000 mcg/mL = 0.10 mL. Units: 0.10 × 100 = 10 units.

Larger vial and larger dose, same draw as Example 1 — the math balances out.

Why bac water volume changes everything

The bac water volume you add is not fixed. It's your choice, and it sets the concentration for every dose from that vial. If you add more bac water:

• Concentration goes down (more dilute).
• You draw more units per dose.
• Measurements are easier to read on the syringe — less precision required.

If you add less bac water:

• Concentration goes up (more potent per unit).
• You draw fewer units per dose — small errors matter more.
• Each dose occupies less injection volume, which some people prefer.

There is no universally correct amount. Most protocols use 1–3 mL depending on vial size and dose. My Pep Calc stores the bac water amount per vial so the calculation is always based on the right number.

Syringe types and the ×100 conversion

This guide assumes a U100 insulin syringe (the most common type for peptide injections). U100 means 100 syringe units = 1 mL.

If you use a U40 syringe (40 units = 1 mL, common in some countries), replace the ×100 with ×40 in the formula. Drawing the same volume with a U40 syringe reads a different number.

If you use a 1 mL tuberculin syringe (graduated in hundredths of a mL, no "units"), skip the syringe conversion entirely and just draw the volume in mL directly.

The My Pep Calc reconstitution calculator handles all three syringe types. Select yours before reading the result.

Common mistakes

Forgetting to convert mg → mcg

Vials are labeled in milligrams. Doses are described in micrograms. 1 mg = 1,000 mcg. If you skip the conversion, your result is off by a factor of 1,000 — you will either draw nothing meaningful or an enormous amount. Always convert to the same unit before dividing.

Using the wrong bac water amount

If you added 2 mL but calculated with 1 mL, every dose is double what you intend. Write the bac water amount on the vial label or store it in My Pep Calc so you can't misremember it.

Shaking the vial

Don't shake. Roll or swirl slowly. Shaking can denature peptide bonds and create bubbles that make precise draws harder. Most peptides dissolve in 30–60 seconds of gentle swirling; some take several minutes.

Injecting bac water directly at the powder

Aim the bac water stream at the glass wall, not the powder. Direct high-pressure injection onto the powder can also damage peptide structure.

Storage after reconstitution

Reconstituted peptides should be refrigerated at 2–8 °C (35–46 °F). Most are stable for 28–56 days refrigerated with benzyl alcohol-preserved bac water. Do not freeze a reconstituted solution. Store away from light.

Lyophilized (unreconstituted) powder is more stable and can typically be stored at room temperature for months, or refrigerated for longer. Reconstitute only what you need in the near term.

Why tracking the vial matters as much as the math

The reconstitution calculation assumes the vial still contains its original concentration. But every time you draw a dose, the volume in the vial decreases. The concentration stays the same (you're removing solution in proportion), but the total remaining doses change.

My Pep Calc tracks doses remaining per vial so you know when you'll run out — and flags if your draw would empty the vial before the protocol calls for a new one. A reconstitution calculator that doesn't track inventory is half a tool.