Reconstitution math without the spreadsheet: bac water, units, and what the syringe actually shows

I get some version of this question from almost everyone starting their first peptide protocol: a vial shows up with a number in milligrams, the bac water bottle shows milliliters, and the syringe shows units, and none of those three numbers talk to each other directly. Short answer: reconstitution math is three steps, not a spreadsheet, and once you've done it twice by hand you won't need to look it up again. This post walks through those three steps using BPC-157 as the running example, then shows how the same math applies to GLP-1 peptides like semaglutide and retatrutide.

Step one: turn milligrams and milliliters into a concentration

Every vial of lyophilized (freeze-dried) peptide has a total amount of peptide in it, printed on the label in milligrams. When you add bacteriostatic water, you're not changing how much peptide is in there, you're just dissolving it into a liquid you can measure with a syringe. The concentration is simply the total milligrams divided by the milliliters of water you added.

Take a 10 mg vial of BPC-157. Add 2 mL of bacteriostatic water and you get 10 mg ÷ 2 mL = 5 mg/mL. That's it, that's the whole first step. If you'd used 1 mL instead, you'd have 10 mg/mL, twice as concentrated, and every dose afterward would use half the volume for the same amount of peptide. If you used 5 mL, you'd have 2 mg/mL, a much weaker solution that gives you more room to dial in small doses precisely.

I always convert milligrams to micrograms before I go further, because most peptide doses in this space are written in micrograms (mcg), and mixing units is where people make mistakes. 5 mg/mL is the same thing as 5,000 mcg/mL. Write it down that way and the rest of the math gets easier.

Step two: figure out what volume gets you your target dose

Once you know your concentration in mcg/mL, your target dose divides cleanly into a volume. Say you want a 500 mcg dose of BPC-157, which sits right in the middle of the range most people run, according to the community-reported protocols on the BPC-157 encyclopedia entry. At 5,000 mcg/mL, you need 500 ÷ 5,000 = 0.1 mL.

Here's where the confusion usually comes in: 0.1 mL doesn't look like a real amount you can see on a syringe. That's what step three is for.

Step three: convert milliliters to the units on your syringe

The insulin syringes most people use for subcutaneous peptide injection are marked in units, not milliliters, and a standard U-100 syringe holds 100 units per 1 mL. That means 1 unit = 0.01 mL. To convert, multiply your target volume by 100.

0.1 mL × 100 = 10 units. So on that 10 mg vial reconstituted with 2 mL of bac water, a 500 mcg dose is 10 units on the syringe. If your target dose were 250 mcg instead, you'd land on 5 units. This same three-step process, milligrams over milliliters to get concentration, target dose over concentration to get volume, volume times 100 to get units, works for any peptide and any vial size. The BPC-157 encyclopedia page and the human trials behind semaglutide dosing (the reconstitution logic is identical there too, at PMID 33567185, the STEP-1 trial) both work on the same arithmetic, just with different target numbers.

I keep a small card taped inside my supplement cabinet with this math written out for the two or three peptides I run regularly, because even after doing it a hundred times, I don't trust myself to do it from memory at 6 AM before coffee. There's no shame in writing it down. The people who get into trouble are the ones who guess.

A worked example with a different vial size

Let's say you've got a 5 mg vial instead of 10 mg, and you reconstitute with 2 mL. That's 5 mg ÷ 2 mL = 2.5 mg/mL, or 2,500 mcg/mL. A 250 mcg dose is 250 ÷ 2,500 = 0.1 mL, which is 10 units. Notice that's a different number of units for the same 250 mcg dose than you'd get from the 10 mg vial, because the concentration changed. This is exactly why copying someone else's "I draw 10 units" advice without knowing their vial size and water volume is how people end up over- or under-dosing without realizing it.

For retatrutide, the math scales the same way but the doses run higher. A 10 mg vial with 2 mL of bac water gives you 5 mg/mL, and a 2 mg starting dose (a common conservative starting point discussed on the retatrutide encyclopedia page) works out to 0.4 mL, or 40 units. Same three steps, bigger numbers.

Handling the vial and storing it once mixed

A couple of practical notes that aren't strictly math but matter for getting an accurate draw every time. Add the bacteriostatic water slowly, running it down the inside wall of the vial rather than blasting it straight into the powder, and then swirl gently to dissolve. I want to be direct about something here because there's a persistent myth in peptide forums: shaking a vial does not denature BPC-157 or similarly sized peptides. The reason people say swirl instead of shake is about foam, which makes it harder to draw an accurate dose, not about protecting the molecule from damage. The things that actually degrade a reconstituted peptide are heat, time spent dissolved in solution, light exposure, oxidation, and repeated freeze-thaw cycles, not agitation. If you want the deeper breakdown on what actually threatens peptide stability once mixed, the site's freeze-thaw and storage piece covers that ground well.

Store the reconstituted vial in the fridge, and most practitioners treat a properly stored bacteriostatic-water solution as good for around 30 days. If it ever turns cloudy or you see particles floating in it, throw it out rather than second-guessing yourself.

Now the part my lawyer makes me say, and he's right to: the doses and schedules here are for educational and informational purposes only. These peptides are sold for research use only and are not FDA-approved drugs. This is not medical advice. Consult a qualified physician before beginning any protocol.

Why this matters more than it seems like it should

I've watched people get so anxious about the math that they either avoid starting a protocol altogether or, worse, guess at a dose because they didn't want to admit they were confused. Neither outcome serves you. The math itself is genuinely simple once you separate it into the three steps above, and it's the same three steps every time regardless of which peptide you're working with. What changes is the vial size, the water volume, and the target dose, none of which changes the underlying arithmetic.

The evidence backing BPC-157's use in tissue repair goes back to foundational work like the 1994 gastric-protection studies that first characterized the compound, and the mechanism has been mapped in more detail since, including how it activates the cell-migration pathways that let repair cells reach an injury site. None of that research changes because you got your reconstitution math wrong, but an inaccurate dose does undercut your ability to actually see the effect the research describes. Getting the arithmetic right is how you make sure the peptide you're using is doing the job the studies say it can do.

If you're stacking BPC-157 with other compounds, or moving between a GLP-1 protocol and a repair-focused one, the auto-injector pens covered in this workflow guide can also simplify things once you've nailed down the underlying dose, since they remove some of the manual draw-and-measure step. But even with a pen, knowing the math behind your dose means you can catch a mistake before it becomes one.