What reconstitution is
Many peptides are supplied as a dry powder or a small white cake at the bottom of a sealed vial. The powder is made by freeze-drying, also called lyophilization. Peptides tend to break down in water over time, so removing the water lets a product survive shipping and storage.
Reconstitution is the reverse step: a measured volume of liquid, the diluent, is added to the powder to turn it back into a solution. From a math point of view, one thing happens at that moment. The vial gains a concentration: a known amount of peptide spread through a known volume of liquid. Every later number, including the mark on a syringe, comes from that concentration.
That is also why two vials of the same product can need different syringe marks for the same amount. If they were reconstituted with different volumes, they have different concentrations.
Bacteriostatic water vs sterile water
The two diluents that come up most often are both FDA-labelled products, and their labels describe the difference precisely.
| Bacteriostatic water | Sterile water | |
|---|---|---|
| Full name | Bacteriostatic Water for Injection, USP | Sterile Water for Injection, USP |
| Preservative | Benzyl alcohol, 0.9% or 1.1% | None |
| Container | Multiple-dose: repeated withdrawals may be made | Single-dose only: the label says to discard the unused portion |
| Also on the label | Not for use in newborns | Do not reuse single-dose containers |
“Bacteriostatic” means the benzyl alcohol inhibits the growth of bacteria. It does not sterilize, and it does not make a vial last indefinitely. “BAC water” is simply the common abbreviation. The label also notes that the product is meant only for diluting or dissolving drugs, and that injecting it into a vein on its own, without a drug dissolved in it, may damage red blood cells.
For opened multiple-dose vials in general, the CDC’s guidance for healthcare settings is that the vial should be dated and discarded within 28 days unless the manufacturer states another date. That guidance concerns vials such as the diluent. It says nothing about how long a particular reconstituted peptide remains stable, which depends on the product. For many research peptides, no stability data has been published at all.
Which diluent belongs with a product is not a matter of preference. Approved freeze-dried medicines name their diluent in the labelling, and some include it in the box. The labelling and your pharmacist decide, not a guide like this one. The BAC water calculator only does the volume arithmetic, whichever liquid is involved.
Reading a vial label
The number the math needs is the total amount in the vial, usually printed in milligrams (mg): 2 mg, 5 mg, 10 mg and so on. A few things are worth noticing.
- It is a total, not a concentration. A dry vial has no mg/mL until liquid is added.
- Some vials are labelled in IU (international units) instead of mg. IU measures biological activity and is defined separately for each substance, so there is no general IU-to-mg conversion. IU on a vial label is also unrelated to the “units” printed on an insulin syringe.
- Blend vials hold two or more compounds. The label may give a combined total or an amount for each. The blend calculator works each component out separately.
- The math takes the label at its word. No calculator can tell you whether a vial really contains what its label says.
Insulin syringe types and how they are marked
Small volumes are usually measured with insulin syringes, and these are marked in units, not millilitres. A unit on the barrel is a measure of volume. It was designed around insulin: a U-100 syringe is made for insulin at 100 units per mL, so its scale divides 1 mL into 100 parts.
U-100 syringes come in three common barrel sizes:
| Barrel | Holds | Usual graduation |
|---|---|---|
| 1 mL | 100 units | Every 2 units |
| 0.5 mL | 50 units | Every 1 unit |
| 0.3 mL | 30 units | Every 1 unit, sometimes every half unit |
On all three, one unit is the same 0.01 mL. The smaller barrels simply spread the scale out, which makes small volumes easier to read. Check the graduation before reading a mark: on a 1 mL barrel each small line is usually two units, not one.
U-40 syringes are a different scale, with 40 units per mL, so one unit is 0.025 mL. They are mostly sold for veterinary insulin. The same mark on a U-40 and a U-100 barrel is a very different volume, so check which one is printed on the barrel. The calculators on this site assume U-100.
Tuberculin syringes also hold 1 mL but are marked in millilitres, in steps of 0.01 mL, with no unit scale at all.
The reconstitution math
There are only three steps, and each is a single multiplication or division.
1. Concentration
The reconstitution calculator does this step, and also shows the result in mcg/mL and per syringe unit.
2. Volume for an amount
Keep both in the same unit. If the amount is in mcg, use the concentration in mcg/mL.
3. Syringe units
The peptide calculator runs all three steps together, and the dosage calculator converts an amount you enter into mL and units, or a syringe mark back into an amount.
The reverse question
Sometimes the question runs the other way: what volume of liquid would make a chosen amount land on a chosen, easy-to-read mark?
That is what the BAC water calculator works out. Both amounts are values you supply. The tool does not choose them.
Worked example of the arithmetic
These numbers are arbitrary. They were picked because they divide cleanly, they are not tied to any compound, and they are not a suggestion of what to use.
- A vial labelled 5 mg has 2 mL of liquid added. 5 ÷ 2 = 2.5 mg/mL.
- In micrograms: 2.5 × 1,000 = 2,500 mcg/mL.
- Each unit on a U-100 syringe is 0.01 mL, so one unit holds 2,500 ÷ 100 = 25 mcg.
- An amount of 250 mcg is 250 ÷ 2,500 = 0.1 mL, and 0.1 × 100 = 10 units.
Now change only the liquid. The vial still holds 5 mg, but every other number moves:
| Liquid added | Concentration | Per unit (U-100) | 250 mcg is |
|---|---|---|---|
| 1 mL | 5 mg/mL | 50 mcg | 5 units |
| 2 mL | 2.5 mg/mL | 25 mcg | 10 units |
| 2.5 mL | 2 mg/mL | 20 mcg | 12.5 units |
More liquid does not mean less peptide. It means the same amount in a larger volume, so each unit mark holds less. It also shows why a volume matters for readability: 12.5 units cannot be read exactly on a barrel graduated every 2 units.
Unit conversions
| From | To | How |
|---|---|---|
| mg | mcg | Multiply by 1,000 |
| mcg | mg | Divide by 1,000 |
| mL | U-100 units | Multiply by 100 |
| U-100 units | mL | Divide by 100 |
| mL | cc | The same thing: 1 mL = 1 cc |
| IU | mg | No general conversion. It is specific to each substance. |
A microgram is written mcg or μg. Both mean one thousandth of a milligram.
Common slips the math can catch
- Mixing up mg and mcg. The two differ by a factor of 1,000, and it is the easiest slip to make when a label is in mg and an amount is in mcg.
- Treating syringe units as an amount. Units are volume. Ten units from two vials with different concentrations are two different amounts.
- Reusing an old number for a new vial. A unit mark is only valid for the concentration it was worked out from.
- Reading a U-40 barrel as U-100, or counting each line on a 1 mL barrel as one unit when it is two.
- Expecting a mark the barrel does not have. The calculators flag results that do not land on a whole unit.
These are not hypothetical. In July 2024 the FDA warned about dosing errors with compounded semaglutide supplied in multiple-dose vials. Reports described people drawing up 5 to 20 times the intended amount, often after confusing milligrams, millilitres and syringe units. Arithmetic can catch a slip like that, but it cannot tell you whether the intended amount was right in the first place. Have a pharmacist or prescriber confirm any measurement.
What to write down
Most of the slips above come from memory: a concentration worked out weeks ago, or a mark carried over from a different vial. A written record fixes that. For each vial, it helps to note:
- The name on the label, the labelled amount, and a lot number if one is printed
- The date it was reconstituted
- Which liquid was added, and exactly how much
- The concentration that results
And for each entry: the date and time, the amount, the syringe mark, the injection site and any notes. A record like this is also what makes a conversation with a healthcare provider accurate.
This is the job Peptrack does. It saves each vial’s size and the water added, keeps count of what is left as you log, and reminds you on the schedule you set. It is a free peptide tracker with no paid tier. See the features, or start from the homepage.
What this guide is not
- It is not preparation instructions. It does not cover sterile technique, how to add liquid to a vial, or how to inject.
- It does not suggest a dose, a schedule or a volume of liquid for any compound. The same goes for the per-compound calculators, which carry reference facts and arithmetic only.
- It does not say how to store a reconstituted product or how long it keeps. That is product-specific and belongs to the labelling and your pharmacist.
- It is not medical advice, and it does not replace a licensed healthcare provider. Read the full disclaimer.
Sources
Statements about diluents, vials and the FDA warning come from these primary sources, last checked in September 2026.
