Fundamentals
Peptide reconstitution, explained
Last reviewed · Written by the PepCycle team
Reconstitution is dissolving a freeze-dried peptide in a liquid so it can be drawn into a syringe. The arithmetic is three steps: divide the vial's mass by the water volume to get concentration; divide your intended amount by that concentration to get millilitres; multiply by 100 for U-100 syringe units. Nothing about it is medical — it is division.
What reconstitution is
Research peptides are shipped lyophilised — freeze-dried into a dry pellet — because a dry peptide is far more stable than one sitting in solution. Reconstitution is simply putting the water back: a measured volume of diluent is added to the vial, the powder dissolves, and the result is a solution of known concentration.
The critical consequence is that the concentration is not a property of the vial you bought. A 5 mg vial is 5 mg of peptide whether you dissolve it in 1 mL or 5 mL. You decide the concentration at the moment you add water, and every subsequent calculation depends on that decision being recorded.
- Bacteriostatic water
- Sterile water containing 0.9% benzyl alcohol as a preservative, which inhibits bacterial growth and is why it is used for vials accessed more than once. Plain sterile water contains no preservative and is intended for single use.
The three-step arithmetic
Step 1 — Concentration
concentration = vial mass ÷ diluent volume. A 5 mg vial with 2 mL of bacteriostatic water gives 2.5 mg/mL, which is 2,500 mcg/mL.
Step 2 — Volume
volume = amount ÷ concentration. If you intend to draw 250 mcg from a 2,500 mcg/mL solution, that is 0.1 mL.
Step 3 — Syringe units
units = volume in mL × 100 on a U-100 insulin syringe, because 100 units = 1 mL. So 0.1 mL is 10 units.
Worked examples
| Vial | Water added | Concentration | 1 unit contains |
|---|---|---|---|
| 5 mg | 1 mL | 5,000 mcg/mL | 50 mcg |
| 5 mg | 2 mL | 2,500 mcg/mL | 25 mcg |
| 5 mg | 5 mL | 1,000 mcg/mL | 10 mcg |
| 10 mg | 2 mL | 5,000 mcg/mL | 50 mcg |
| 100 mg | 5 mL | 20,000 mcg/mL | 200 mcg |
Notice rows one and four: a 5 mg vial in 1 mL and a 10 mg vial in 2 mL are identical in every practical sense. The ratio is what matters, not either number alone.
Where the errors come from
- Confusing mg and mcg. A factor of 1,000 is the most consequential typo available. Whole-number entry in a single unit avoids most of it.
- Treating units as an amount. "I take 10 units" is only meaningful alongside "from a 5 mg vial in 2 mL". Written down separately, the two facts drift apart.
- Re-diluting a vial. Topping up a partly used vial with more water changes the concentration and invalidates every earlier unit-to-amount conversion for it. Record it as a new vial.
- Assuming the syringe. U-100 is the common insulin syringe, but U-40 and U-50 syringes exist and the marks mean different things. See reading an insulin syringe.
- Ignoring displacement. For most research peptides the powder's own volume is negligible; for very large-mass vials it is a small source of imprecision worth being aware of rather than pretending away.
What to record
- Vial mass as printed, and the exact diluent volume added.
- Diluent type — bacteriostatic or plain sterile water.
- Reconstitution date, which starts the in-use clock.
- Derived concentration, so future entries need no mental arithmetic.
- Syringe type in use, since the unit reading depends on it.
In PepCycle, entering the vial mass and diluent volume once means every later entry converts automatically in both directions — enter micrograms and see units, or enter units and see micrograms.
Frequently asked questions
How do I calculate peptide reconstitution?
Divide the vial's peptide mass by the volume of bacteriostatic water added to get the concentration. Divide the amount you intend to draw by that concentration to get the volume in millilitres. Multiply by 100 for units on a U-100 insulin syringe. That is the complete calculation.
How much bacteriostatic water should I add to a vial?
That is a choice, not a rule, and this site will not pick a number for you. More water gives a more dilute solution with larger, easier-to-read volumes; less water gives a concentrated solution with smaller volumes. Whatever you choose, record it — that number is what makes every later entry interpretable.
Is one insulin unit the same as one microgram?
No, and treating them as equivalent is the most common serious error in peptide logging. A unit on a U-100 syringe is 0.01 mL of volume. How many micrograms sit in that volume depends entirely on your vial's concentration.
Can I add more water to a vial I have already started?
Doing so changes the concentration, which means every earlier conversion of units to micrograms for that vial no longer applies. If you do it, close the old vial record and create a new one so your history stays interpretable.
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