What this does
Reconstitution is division: milligrams in the vial ÷ milliliters of diluent gives concentration, and dose ÷ concentration gives the volume you draw. Multiplying that volume by 100 gives the marking on a U-100 insulin syringe. The calculator runs those three steps and shows the working, so you can check it rather than trust it.
A note on what this calculator is not. It converts between milligrams and millilitres. It does not know what dose is appropriate for anything, because that is not an arithmetic question, and for most research peptides there is no established human dose to look up. It assumes you already have a number and need the volume that corresponds to it.
The three-step formula
All of reconstitution arithmetic is these three lines. Worth knowing them, if only to catch a calculator having a bad day.
| Step | Formula | Worked example |
|---|---|---|
| Concentration | mg in vial ÷ mL of diluent | 10 mg ÷ 2 mL = 5 mg/mL |
| Volume per dose | dose (mg) ÷ concentration | 0.5 mg ÷ 5 mg/mL = 0.1 mL |
| Syringe marking | volume (mL) × 100 | 0.1 mL × 100 = 10 units |
Units measure volume, not dose. This is the single most common and most dangerous misreading in the whole area. A U-100 insulin syringe is graduated so 100 units = 1 mL. The marking tells you how much liquid you drew, and says nothing about how much peptide was dissolved in it. Ten units of a 5 mg/mL solution is 0.5 mg. Ten units of a 1 mg/mL solution is 0.1 mg. Same marking, five-fold difference. Anyone who tells you a dose “in units” without stating the concentration has not given you a dose.
Choosing how much diluent to add
There is no correct answer, only a trade-off, and the calculator makes it visible: add more water and each dose becomes a larger volume that is easier to measure accurately, but the solution is more dilute and you may run out of vial space. Add less and the solution is concentrated, but doses can shrink below the smallest readable marking on the syringe.
A practical target is a dose that lands somewhere between roughly 10 and 50 units on a U-100 syringe. Below about 5 units, syringe reading error becomes a large fraction of the dose. Above 50 units you are drawing a lot of liquid, and with a 1 mL syringe you may not fit the dose at all — the calculator flags both cases.
Milligrams and micrograms
1 mg = 1,000 mcg. Peptides dosed in the hundreds of micrograms — several of the growth-hormone secretagogues, for instance — are where this bites, because it is genuinely easy to slip a factor of a thousand when a vial is labeled in milligrams and a protocol is written in micrograms. The calculator asks for the unit explicitly rather than inferring it, for exactly this reason.
What the calculator can't tell you
The arithmetic is the easy part. The parts that actually determine whether a reconstituted vial is any good:
- Diluent choice. Bacteriostatic water contains 0.9% benzyl alcohol as a preservative and is what allows a vial to be used more than once. Sterile water has no preservative. The difference matters.
- Technique. Diluent runs down the vial wall rather than jetting onto the powder; the vial gets swirled, not shaken. Why those two rules exist comes down to shear and the air-liquid interface.
- Stability after reconstitution. A lyophilized peptide is stable for a long time; the same peptide in solution is on a clock. Storage and stability covers what that clock depends on.
- Whether the vial contains what the label says. No calculator can tell you that. A batch-matched certificate of analysis can.
Frequently asked questions
How do you calculate peptide reconstitution?
Divide the milligrams of peptide in the vial by the millilitres of bacteriostatic water you add. That gives concentration in mg/mL. To find the volume for a given dose, divide the dose (in mg) by that concentration. A 10 mg vial with 2 mL of water is 5 mg/mL, so a 0.5 mg dose needs 0.1 mL.
How many units is 0.1 mL on an insulin syringe?
10 units. A U-100 insulin syringe is graduated so that 100 units equals 1 mL, so units are simply millilitres times 100. Critically, that marking measures volume, not dose — 10 units of a 5 mg/mL solution and 10 units of a 2 mg/mL solution are very different amounts of peptide.
How much bacteriostatic water should I add to a vial?
There is no single correct answer — it is a trade-off you choose. More diluent makes each dose a larger, easier-to-measure volume but leaves the solution more dilute. Less diluent concentrates it, which risks doses too small to read on the syringe. Most people pick a volume that makes their intended dose land somewhere between 10 and 50 units on a U-100 syringe.
Does the peptide powder add to the volume?
Negligibly at these quantities. A few milligrams of lyophilized powder displaces a volume too small to matter against 1–3 mL of diluent, so the calculation treats total volume as equal to diluent volume. This is the standard assumption and the error is far smaller than syringe reading error.
What is the difference between mcg and mg?
1 mg = 1,000 mcg (micrograms). Mixing the two up by a factor of a thousand is the most common and most consequential arithmetic error in this whole area, which is why the calculator makes you pick the unit explicitly rather than guessing.
References
- American Society of Health-System Pharmacists (ASHP). Handbook on Injectable Drugs: Benzyl alcohol preservative compatibility and stability data. https://www.ashp.org/products-and-services/database-tools/handbook-on-injectable-drugs
- United States Pharmacopeia. (2024). Bacteriostatic Water for Injection — monograph and in-use stability guidance. USP-NF. (See institutional access.) https://doi.org/10.4135/9781412963855.n1200
- Wang, W. (1999). Instability, stabilization, and formulation of liquid protein pharmaceuticals. International Journal of Pharmaceutics, 185(2), 129–188. https://doi.org/10.1016/s0378-5173(99)00152-0
The arithmetic needs no citation; the diluent and stability guidance does.