How to Reconstitute Peptides and GLP-1s: The BAC Water, mg and Units Math, Explained

Quick answer: divide the mg in the vial by the mL of liquid you added to get the concentration (mg/mL). Divide the amount you want to draw by that concentration to get mL, then multiply by 100 for units on a U-100 insulin syringe. For example, 10 mg in 2 mL is 5 mg/mL, so 1 mg is 0.2 mL, or 20 units. Plug your own numbers into the calculator below; the formula and more worked examples follow.

If you're trying to work out how to reconstitute peptides, you've probably stared at a vial of powder, a bottle of bacteriostatic water and an insulin syringe marked in "units" and wondered how they add up to your dose. You're not alone. Peptide reconstitution math is one of the most common sources of confusion for people managing peptide or compounded GLP-1 protocols. The good news: it's one simple formula, and once you see it worked through a few times, it stops feeling like guesswork.

Educational only, not medical advice. This guide explains arithmetic and general handling concepts. It does not recommend a dose, a dilution, or any compound. Always follow your prescriber's instructions and the product label, and ask a pharmacist if anything is unclear.

Units for common amounts, by vial size

Math only, using 2 mL of liquid as the example volume. The amounts are round numbers, not suggestions. Results such as 3.3 units usually fall between syringe marks; see how to read insulin syringe tick marks.

Other water volumes and the 10 mg vial in detail: how much BAC water for a 10 mg vial. Not sure whether your label shows a total or a strength? how to read a vial label.

What does reconstituting a peptide mean?

Many peptides and some compounded medications ship as a freeze-dried (lyophilized) powder. Before the powder can be drawn into a syringe, it has to be dissolved in a sterile liquid, most often bacteriostatic water (BAC water), which is sterile water with a small amount of benzyl alcohol added as a preservative.

Two things matter:

1. How much active ingredient is in the vial (printed on the label, usually in mg).
2. How much liquid you add (in mL).

Together, those set the concentration: how many mg are in each mL of the mixed solution. Every dose calculation after that depends on this one number.

One important note: branded, ready-to-use GLP-1 pens and pre-filled products are not reconstituted. If your medication arrives as a liquid or in a pen, follow its label and don't add anything to it.

How to reconstitute peptides step by step

This is the general sequence most product instructions follow. Your prescriber's, pharmacist's or product's instructions always come first, including how much liquid to add and which liquid to use.

1. Check the label and your instructions. Note the amount in the vial (mg), which diluent you've been told to use, and how much of it.
2. Gather clean supplies. The vial, the specified diluent, a new sterile syringe and needle, alcohol swabs, and a sharps container.
3. Wash your hands and swab both stoppers with alcohol, and let them air-dry.
4. Draw the instructed volume of diluent into the syringe.
5. Add it slowly down the inside wall of the vial rather than spraying it straight onto the powder.
6. Swirl or roll gently until dissolved. Don't shake hard. Some powders take a few minutes.
7. Look before you use it. The solution should usually be clear. If it's cloudy, has particles or looks discolored, stop (see below).
8. Label the vial with the date you mixed it, the volume you added and the resulting concentration.
9. Store it as the label directs, usually in the refrigerator.

Never reuse or share syringes or needles, and use a new sterile syringe for every puncture.

Specific to GLP-1s: how to reconstitute semaglutide and how to reconstitute tirzepatide.

The three units you need to keep straight

Most mistakes come from mixing up these three:

• mg (milligrams): the amount of active ingredient.
• mcg (micrograms): also an amount, just smaller. 1 mg = 1,000 mcg.
• Syringe "units": a volume marking, not an amount of drug. On a U-100 insulin syringe, 100 units = 1 mL, so 1 unit = 0.01 mL.

That last point is the big one. "10 units" doesn't mean the same dose in every vial. It means 0.1 mL of liquid, and how much drug is in that 0.1 mL depends entirely on how concentrated your vial is.

Check the barrel, too. The math in this guide assumes a U-100 syringe. U-40 and other syringe types exist, and their markings are not 0.01 mL per unit. Our guide to insulin syringe units, U-100 vs U-40 and reading the tick marks covers the differences.

How do you convert mg to units on an insulin syringe?

Here's the whole thing:

1. Concentration (mg/mL) = mg in vial ÷ mL of water added
2. Volume to draw (mL) = dose (mg) ÷ concentration (mg/mL)
3. Units on a U-100 syringe = volume (mL) × 100

You can also run it backwards to convert units to mg and double-check a draw: units ÷ 100 × concentration = mg. The calculator at the top of this guide does all three steps, and it also lives on its own page as the peptide reconstitution calculator.

If your prescribed amount is in mcg, convert it to mg first (divide by 1,000), or convert the concentration to mcg/mL (multiply by 1,000). Just keep both numbers in the same unit. Our mg to mcg conversion guide has charts for both.

One more thing worth knowing: the formula trusts the mg printed on the label. If you want to understand what a lab report says about how much peptide is really in a vial, see our guide on how to read a peptide COA, including net peptide content.

Worked examples

The numbers below are illustrations of the arithmetic only. They are not dose suggestions for any product.

Example 1: 10 mg vial + 2 mL water

• Concentration: 10 mg ÷ 2 mL = 5 mg/mL
• Drawing 0.1 mL gives you 0.1 × 5 = 0.5 mg
• 0.1 mL on a U-100 syringe = 10 units

So in this vial, 10 units = 0.1 mL = 0.5 mg.

Example 2: Same vial, different water

Now take the same 10 mg vial but add 1 mL of water instead:

• Concentration: 10 mg ÷ 1 mL = 10 mg/mL
• 10 units (0.1 mL) now contains 1 mg, double Example 1.

This is why "how many units do you take?" is a meaningless question without also knowing how much water went into the vial. Same powder, same syringe mark, twice the amount of drug.

Example 3: Working in mcg

Say a vial contains 5 mg and is mixed with 2 mL of water, and the amount on your instructions is written in mcg.

• Concentration: 5 mg ÷ 2 mL = 2.5 mg/mL = 2,500 mcg/mL
• For an amount of 250 mcg: 250 ÷ 2,500 = 0.1 mL
• 0.1 mL × 100 = 10 units

Tracking it in Caliper

Doing this math in your head at the counter is where errors creep in. Work it out with the reconstitution calculator on this site — a browser tool, not part of the app — then save the vial in Caliper so its strength, water and concentration are recorded alongside every dose you log. Caliper records what you enter; it doesn't recommend or calculate a dose. Set up your first vial in Caliper.

How much BAC water should you add to peptides?

There's no single "right" amount of water for every product, and that's exactly why the question comes up so often. The volume you add changes the concentration, not the total amount of drug in the vial. The practical trade-offs:

• More water = a more dilute solution, so each dose is a larger volume (more units). That can make small doses easier to measure accurately.
• Less water = a more concentrated solution, so each dose is a smaller volume. Small measuring errors make up a bigger share of the dose.

Your prescriber, pharmacist or the product's instructions should tell you how much diluent to use, and sometimes which diluent. Some products call for something other than BAC water. Don't swap diluents or mix in additives on your own. Whatever you use, write the volume down, because every future dose depends on it. For a side-by-side look at what 1, 2, 3 or 5 mL does to a 10 mg vial, see how much BAC water to add to a 10 mg vial; for why more water means more units for the same mg, see does adding more BAC water change your dose? If one vial holds two compounds, the math runs once per component: see reconstituting a blend vial. And if you're working with tirzepatide, check first whether it needs mixing at all: how to reconstitute tirzepatide.

Cloudy, burning or unsure: when to stop and ask

Peptide cloudy after reconstitution

A properly reconstituted solution is usually clear. Cloudiness, particles, or discoloration can signal a problem, such as contamination, the product not dissolving properly, or the wrong diluent. Don't try to "fix" it by adding other liquids. Don't use it; contact your pharmacist or prescriber. Our post on why a peptide is cloudy after reconstitution covers the causes and what to do.

Does BAC water burn?

Stinging at the injection site is common to ask about. Possible contributors include the preservative in BAC water, the solution's acidity, a cold solution, injection volume, or technique. Persistent or worsening pain, redness, swelling, heat, or any sign of infection is a reason to check with your prescriber rather than push through. Our post on why injections burn or sting looks at the research on each of these factors, including the preservative.

Logging which site you used and how it felt makes these conversations much easier. Caliper includes injection-site rotation tracking and lets you log symptoms alongside your doses, so you can show your provider exactly what happened and when.

Storage: do reconstituted peptides need to be refrigerated?

Keep this conservative and always follow the label:

• Reconstituted peptides are generally stored in the refrigerator, protected from light, and not frozen unless the label says otherwise.
• Unopened BAC water is typically stored at room temperature per its label. Once a multi-dose vial is punctured, its label usually gives a limited use-by window. Respect it.
• How long do reconstituted peptides last? It varies by product, so there's no universal number. Use the use-by period on your label, and ask your pharmacist if your label doesn't say. Don't rely on a figure from a forum or a different product. Our guide to storing peptides and GLP-1s explains beyond-use dates and the 28-day multi-dose rule.

Caliper lets you log each vial with its reconstitution and use-by dates, so you're not guessing how old a vial is.

FAQ

How do you reconstitute peptides?

Follow your product's instructions: swab the stoppers, add the instructed volume of the specified diluent slowly down the side of the vial, swirl gently until dissolved, check that it's clear, then label and store it as directed. Your concentration is the mg in the vial divided by the mL you added.

How much bacteriostatic water do I add to a 10 mg vial?

2 mL of BAC water makes 5 mg/mL in a 10 mg vial, the volume most often used in examples; 1 mg then takes 20 units on a U-100 syringe. That's arithmetic, not a recommendation: your prescriber, pharmacist or product instructions set the volume for your vial. Other volumes change only the concentration (1 mL makes 10 mg/mL), never the 10 mg in the vial. See how much BAC water for a 10 mg vial for the full table.

How many units is 1 mL on an insulin syringe?

On a U-100 insulin syringe, 100 units = 1 mL, so 1 unit = 0.01 mL. Check that your syringe is U-100 before using that conversion.

How do I convert mg to units on an insulin syringe?

Divide the dose in mg by your vial's concentration in mg/mL to get mL, then multiply by 100 for U-100 units. To go from units back to mg, divide units by 100 and multiply by the concentration.

I added "100 units" of BAC water. How much is that?

On a U-100 syringe, 100 units = 1 mL. So 100 units of water in a 10 mg vial gives 10 mg/mL.

Someone said they take "30 units." Should I?

Not necessarily. Units are volume, so 30 units from their vial can contain a very different amount than 30 units from yours. Your dose comes from your prescriber, and your units come from your vial's concentration.

Does the amount of water change how much drug is in the vial?

No. The total mg stays the same. Water only changes how concentrated it is, and therefore how many units each dose takes.

Can I use sterile water instead of bacteriostatic water?

Only if your product's instructions or your pharmacist say so. Sterile water has no preservative, while BAC water contains benzyl alcohol, and some products call for a different diluent altogether. Don't substitute diluents on your own.

Why is my peptide cloudy after reconstitution?

Cloudiness can mean contamination, incomplete dissolving or the wrong diluent. Don't use it, and don't try to fix it by adding other liquids. Ask your pharmacist or prescriber before doing anything else.

Does BAC water burn when injected?

It can sting for several reasons, including the benzyl alcohol preservative, a cold solution or injection technique. A change after switching supplies is worth mentioning to your pharmacist or prescriber, especially if it's persistent or comes with redness or swelling.

Do reconstituted peptides need to be refrigerated?

Reconstituted peptides are generally refrigerated. Check your product label for the specific storage and use-by guidance.

How long do reconstituted peptides last in the fridge?

It depends on the product. Use the use-by period on your label or from your pharmacist, and log the date you mixed each vial so you always know how old it is.

Can you freeze reconstituted peptides?

Don't freeze a reconstituted vial unless its label specifically says you can.

What is the difference between mg and mg/mL on a vial label?

mg is the total amount in the vial; mg/mL is the strength per mL. 10 mg in 2 mL is 5 mg/mL. See how to read a vial label.

The takeaway

Reconstitution math comes down to one idea: concentration = mg ÷ mL, and on a U-100 syringe, 100 units = 1 mL. Get the concentration right and write it down, and every dose after that is straightforward arithmetic. When something looks or feels off, the right move is always a call to your pharmacist or prescriber, not a guess. And before any of the math, it helps to know what's actually in the vial: our guide to reading and verifying a peptide certificate of analysis covers that side.

Make the math (and the record-keeping) automatic

Caliper is a private, local-first tracker for GLP-1 and peptide protocols. You record the amount you took and it logs each dose with one tap, reminds you when the next one is due, tracks vials with reconstitution and use-by dates, and keeps injection sites, side effects and weight trends on one timeline. Your health data stays on your device, and Caliper never recommends or calculates a dose; it organizes what you and your provider decide.

Start tracking your vials and doses with Caliper →

For adults 18+.

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Medical disclaimer: This article is for general educational purposes only and is not medical advice. It does not recommend any dose, dilution, compound or protocol. Always follow your prescriber's instructions and the product label, and talk to a pharmacist or healthcare provider about any question regarding your medication, its preparation, storage or side effects.