How to use the Peptide Reconstitution Calculator

The tool has three tabs. Start with the first one. The other two answer the questions people usually ask next.

Tab 1: Dose to units

1. Enter the peptide in the vial. Use the amount on the label, in mg.
2. Enter the water you added. This is the total bacteriostatic water in the vial, in mL.
3. Pick your syringe size. All sizes are U-100, but they hold different amounts.
4. Enter your prescribed dose. Choose mcg or mg to match your prescription.
5. Pick how often you inject. The tool uses this to work out how long the vial lasts.
6. Read the results and the notes. You get the unit line, a to-scale syringe picture, doses per vial and days of supply. The notes flag anything that needs a second look.

Tab 2: How much water?

1. Enter the vial size and your dose.
2. Optionally, enter the unit line you would like each dose to land on, such as 10 units.
3. The table compares 0.5 mL to 3 mL of water. It shows which amounts give a dose that is easy to measure and which do not.

Tab 3: Blend vial

1. Enter each ingredient listed on a blended vial, in mg.
2. Enter the water added and the units you draw.
3. The tool shows how much of each ingredient is in that one draw.

How the maths works

Reconstitution maths is three short steps. First find the strength of the liquid. Then find the volume that holds your dose. Then turn that volume into syringe units.

Worked example: a 5 mg vial with 2 mL of water, and a 250 mcg dose.

• 5 × 1,000 ÷ 2 = 2,500 mcg/mL
• 250 ÷ 2,500 = 0.10 mL
• 0.10 × 100 = 10 units

There is also a shortcut. On a U-100 syringe, each unit line holds the concentration divided by 100. At 2,500 mcg/mL, each line holds 25 mcg. So 250 mcg is 10 lines.

How much bacteriostatic water should you add?

There is no single correct amount. If your prescriber, pharmacist or the product leaflet gives a volume, use it. If not, the choice is a trade-off between measuring accuracy and injection volume.

A practical target is a volume that lands each dose on a whole, easy-to-read line, often somewhere between 5 and 30 units. Tab 2 does this for you. It also works out the exact water needed to put your dose on a line you choose.

Example: you want a 250 mcg dose from a 5 mg vial to sit on the 10-unit line. (10 ÷ 100) × (5,000 ÷ 250) = 0.1 × 20 = 2 mL.

Peptide reconstitution chart

Most charts online only show concentration. This one also shows what each unit line holds and how many units 100 mcg takes up. Those are the numbers you need at the syringe.

A quick rule for U-100 syringes: units per 100 mcg = water mL × 10 ÷ vial mg. So 2 mL in a 5 mg vial gives 2 × 10 ÷ 5 = 4 units per 100 mcg.

mcg, mg, units and IU are not the same thing

• mg and mcg measure the amount of peptide. 1 mg = 1,000 mcg.
• Units on a syringe measure volume. On a U-100 syringe, 1 unit = 0.01 mL. How much peptide that holds depends on your concentration.
• IU (international units) measure biological activity. Some products, such as hCG and growth hormone, are labelled in IU. There is no general formula to turn IU into mg. Each product has its own factor, so use the IU figure on your label directly and ask your pharmacist.

Mixing these up is a real danger. In 2024, the US FDA reported dosing errors with compounded semaglutide, caused by confusion between mL, mg and units. Some patients took 5 to 20 times their intended dose. Our units to mL calculator explains how syringe units work in more detail.

Bacteriostatic water vs sterile water

Bacteriostatic water is sterile water with 0.9% benzyl alcohol, a preservative that slows the growth of bacteria. That is why it is used for multi-dose vials that you puncture again and again. The manufacturer’s label says it must not be used in newborns, because benzyl alcohol can be toxic to them.

Sterile water for injection has no preservative. It is meant for single use. Once punctured, it does not protect against bacteria. Some products must be mixed with a specific diluent, so always check the leaflet first.

General reconstitution steps

These are the general clean-technique steps used for powdered injectables. They do not replace the instructions from your pharmacist or the product leaflet.

1. Wash your hands and work on a clean surface.
2. Wipe both rubber stoppers with a fresh alcohol swab and let them dry.
3. Draw the planned amount of bacteriostatic water with a new sterile syringe.
4. Add it slowly, aiming at the glass wall, not straight onto the powder.
5. Swirl or roll gently until dissolved. Do not shake hard. Some peptides are fragile.
6. Check the liquid. If it is cloudy, has particles or looks wrong, do not use it. Ask your pharmacist.
7. Label the vial with the date mixed and the concentration. The “Copy dose card” button in Tab 1 gives you the text.

Storage and the 28-day rule

Follow the storage directions for your product. Many reconstituted peptides are kept in the fridge and away from light, but this varies.

For how long a vial can be used, the FDA’s advice for compounded GLP-1 injections is to discard vials within 28 days of first use, unless the label says otherwise. Tab 1 checks this for you. If your schedule would take longer than 28 days to finish the vial, it tells you. A smaller vial or a different plan from your prescriber may waste less.

Why this calculator has no peptide dose presets

Many peptide calculators include buttons like “BPC-157: 250 mcg”. We left these out on purpose.

• Most peptides sold online have no approved dose for people. The numbers in those presets usually come from forums or seller websites, not from clinical trials.
• A preset can look like a recommendation, even when the page says it is not one.
• The right dose depends on the product, your health and your prescriber. A calculator cannot know any of these.

So this tool does the maths well and leaves the dose to the person who prescribed it.

Safety and legal status

Accurate maths only helps if the product is what the label says. That is the bigger risk with peptides.

• “Research use only” products are not made for people. The FDA has warned that some sellers market unapproved drugs as “for research purposes” or “not for human consumption” while still giving dosing instructions. These products are not checked for purity, strength or sterility.
• Labels can be wrong. The FDA warns that counterfeit or unapproved products may contain too much, too little or none of the active ingredient. If the vial amount is wrong, every number this calculator gives will be wrong too.
• Rules are changing. The FDA’s position on several peptides used in compounding has changed more than once in recent years, including in 2026. What a pharmacy may legally compound can change. Ask your prescriber about the current status of any product.
• Use a new sterile needle and syringe every time, and never share them.
• Get rid of sharps safely. See the FDA’s sharps disposal guidance.
• Seek help straight away if you think you injected too much, or if you have signs of infection or an allergic reaction at the injection site.

This page gives general information only. It is not medical advice, and it does not endorse using any peptide. Please speak to a qualified healthcare professional before starting, mixing or changing any injectable product. See our full disclaimer.

Frequently asked questions

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Sources and further reading

1. US Food and Drug Administration. FDA’s concerns with unapproved GLP-1 drugs used for weight loss. Updated September 2026. fda.gov
2. US Food and Drug Administration. FDA alerts health care providers, compounders and patients of dosing errors associated with compounded injectable semaglutide products. July 2024. fda.gov
3. Pfizer (Hospira). Bacteriostatic Water for Injection, USP: prescribing information. labeling.pfizer.com
4. US Food and Drug Administration. Bulk drug substances used in compounding under Section 503A of the Federal Food, Drug, and Cosmetic Act. fda.gov
5. US Food and Drug Administration. Best way to get rid of used needles and other sharps. fda.gov

Reviewed and updated: September 2026. We update this page when FDA guidance or product labelling changes.