How to reconstitute a peptide vial
Reconstitution means adding a sterile liquid — almost always bacteriostatic water — to a vial of lyophilised (freeze-dried) peptide so the contents can be measured and drawn accurately. The calculator above turns three numbers into a syringe reading:
- Vial contents — the peptide mass printed on the label, in milligrams.
- Bacteriostatic water added — the volume you draw into the vial, in millilitres.
- Target dose — the amount you want in each draw, in micrograms.
From those it returns the vial's concentration, the volume of one dose, and where that dose lands on a U-100 insulin syringe.
The reconstitution math
Every result comes from two steps.
Concentration is the peptide mass divided by the water volume. A 5 mg vial reconstituted in 2 mL is 5 ÷ 2 = 2.5 mg/mL, or 2,500 mcg/mL.
Dose volume is the target dose divided by that concentration. A 250 mcg dose is 250 ÷ 2,500 = 0.1 mL.
A U-100 insulin syringe is marked in units, where 100 units make 1 mL, so 0.1 mL is 0.1 × 100 = 10 units. The calculator also reports micrograms per unit — the figure to re-check against your protocol — and doses per vial, counting whole doses only.
Choosing how much water to add
The water volume does not change how many doses a vial holds; that is fixed by the peptide mass and your dose size. What it changes is how easy the draw is to read.
More water makes each dose a larger mark on the syringe. Reconstituting a 5 mg vial in 2 mL instead of 1 mL turns a 250 mcg dose from 5 units into 10 units, which is easier to measure precisely and gives finer control over small adjustments. A common target is to land a typical dose between 10 and 50 units.
We stock bacteriostatic water in 30 mL vials — the 0.9% benzyl alcohol keeps a reconstituted peptide usable over multiple draws, where plain sterile water is single-use.
Concentration reference
What each vial-and-water combination gives you. This is arithmetic, not a recommendation — the right volume for a given vial depends on the dose you are measuring and how finely you need to read it.
| Vial | Water added | Concentration | Per insulin unit |
|---|---|---|---|
| 5 mg | 1 mL | 5 mg/mL | 50 mcg |
| 5 mg | 2 mL | 2.5 mg/mL | 25 mcg |
| 5 mg | 3 mL | 1.67 mg/mL | 16.7 mcg |
| 10 mg | 1 mL | 10 mg/mL | 100 mcg |
| 10 mg | 2 mL | 5 mg/mL | 50 mcg |
| 10 mg | 3 mL | 3.33 mg/mL | 33.3 mcg |
| 10 mg | 5 mL | 2 mg/mL | 20 mcg |
| 15 mg | 3 mL | 5 mg/mL | 50 mcg |
| 30 mg | 3 mL | 10 mg/mL | 100 mcg |
Check the vial will physically hold the volume before planning around it — most peptide vials are 2–3 mL regardless of the milligrams inside.
The formula
The calculator runs two divisions:
concentration (mcg/mL) = vial contents (mg) × 1000 ÷ water added (mL)
draw volume (mL) = target dose (mcg) ÷ concentration (mcg/mL)
units on a U-100 = draw volume (mL) × 100
Everything else on the page follows from those. Micrograms per unit is the concentration divided by 100; doses per vial is the vial's total micrograms divided by the dose, rounded down, because a partial dose left in the vial is not a dose.
Reading the syringe
The calculator reports both millilitres and insulin units, because which one you need depends on the syringe in front of you.
- U-100 insulin syringe, 1 mL — the usual choice. Graduated 0 to 100 units, one unit per 0.01 mL. A 0.1 mL draw is the 10-unit mark.
- U-100 insulin syringe, 0.5 mL — the same graduation, half the barrel, so it holds 50 units. The marks sit further apart, which makes a small draw easier to read accurately. "U-100" describes the scale, not the capacity.
- 1 mL luer-lock syringe — marked in millilitres directly, so use the mL figure and ignore units. Takes a detachable needle.
- 3 mL luer-lock syringe — marks are typically 0.1 mL, an order of magnitude coarser than an insulin syringe. Fine for reconstituting (drawing the water), poor for measuring a small dose.
Draw the water with whichever syringe is convenient; measure the dose with the finest scale you have.
Frequently asked questions
My dose is in milligrams, not micrograms
Switch Dose in to milligrams and enter it directly. One milligram is 1,000 micrograms, so 0.25 mg and 250 mcg are the same dose — the calculator answers identically either way, and reports the result back in whichever unit you chose. Vial labels are almost always milligrams; published figures are usually micrograms, which is where the confusion comes from.
How do I convert micrograms to insulin units?
Divide your dose by the "per unit" figure the calculator returns. At 2,500 mcg/mL one unit holds 25 mcg, so a 250 mcg dose is 10 units.
I already drew the syringe — how much is in it?
Switch "Solve for" to a mark on the syringe → what it holds, then enter the vial, the water you added, and the mark you drew to. It runs the same arithmetic in the other direction. Useful for checking a vial someone else reconstituted.
Does adding less water give me more doses per vial?
No. Doses per vial is peptide mass ÷ dose size. Water volume only changes the size of the mark you draw to.
Bacteriostatic or sterile water?
Bacteriostatic water contains 0.9% benzyl alcohol, which suppresses bacterial growth and lets a vial be used over several weeks. Sterile water has no preservative and is single-use. For a multi-dose peptide vial, bacteriostatic water is standard.
How long is a reconstituted peptide stable?
It varies by compound, but most are handled for two to four weeks refrigerated after reconstitution. See peptide storage and handling for specifics.
I have a blended vial
Use the blend reconstitution calculator for a vial holding more than one peptide, where a single draw delivers all of them.