Peptide Calculator: Reconstitution & Dosing

How to use this peptide calculator

Three fields do the work: the peptide in the vial (mg), the bacteriostatic water you're adding (mL), and the dose you want. Out comes your concentration, the volume to draw, and the exact units to fill an insulin syringe to — drawn live on a syringe diagram as you type. If you need them, U-40 syringes, half-unit rounding, bodyweight dosing and IU are tucked under More options.

How to reconstitute a peptide, step by step

Reconstituting a peptide just means turning the freeze-dried powder back into a liquid you can measure out. You add bacteriostatic water, the powder dissolves, and the concentration is fixed by two numbers: the milligrams in the vial and the milliliters you added. Nothing else touches it.

1. Draw the water with a 1–3 mL syringe. This one mixes the vial; it isn't the one you inject with.
2. Add it slowly down the vial wall, aiming at the glass rather than straight onto the powder.
3. Swirl gently until clear. Don't shake it.
4. Draw your dose with a U-100 insulin syringe — the calculator tells you which mark.

The two-syringe reality trips up beginners: a bigger syringe mixes the vial once, a U-100 insulin syringe measures each dose after. They are not the same syringe.

The formula, and one worked example

Most peptide calculators are black boxes: you type numbers in, a dose falls out, and you never see the arithmetic. Here's ours in the open. Concentration = peptide ÷ water. Volume = dose ÷ concentration. Units = volume × syringe scale. Collapse those and the whole tool is one line:

units = (D × W) ÷ (P × 10)

D is your dose in micrograms, W the bacteriostatic water in milliliters, P the peptide in the vial in milligrams, on a U-100 syringe. Worked through with the example loaded above: a 5 mgvial in 2 mL of BAC water is 2.5 mg/mL, or2,500 mcg/mL. A 250 mcg dose is 250 ÷ 2,500 =0.10 mL. At 100 units per mL that's 10 units, and the vial holds 20 doses. Straight through the formula: (250 × 2) ÷ (5 × 10) = 10. Show the math in the tool prints these lines for whatever you type.

How much bacteriostatic water should you add?

There's no single correct volume, which catches people off guard. Adding water never changes how much peptide you own, only how spread out it is. 2 mL into a 5 mg vial is a common starting point because it lands a 250 mcg dose on a clean 10-unit mark; add more water and the same dose becomes a bigger, easier-to-read draw. Bacteriostatic water is plain sterile water with 0.9% (9 mg/mL) benzyl alcohol added as a preservative, in a multiple-dose vial you can puncture again and again — straight off theFDA label. More inwhat bacteriostatic water is.

How long does a reconstituted peptide vial last?

We won't hand you a number, because the number everyone repeats isn't measuring what they think. You'll see 28 days quoted all over this topic. It's real, and it has a source: USP ⟨797⟩, the pharmacy compounding standard, sets the beyond-use date for an opened multiple-dose container with antimicrobial preservatives at 28 days "unless otherwise specified by the manufacturer", pointing at ⟨51⟩ — the test for how long a preservative holds bacteria back.

So 28 days describes the benzyl alcohol, not your peptide. It isn't on the bacteriostatic water label at all; we went looking. How long a given peptide stays potent in solution depends on the molecule, and we won't invent an answer — which is what that number gets used for everywhere else.

Reading the insulin syringe: U-100, U-40, and small draws

A U-100 insulin syringe is marked in units, where 100 units = 1 mL, so one unit is 0.01 mL. A 0.3 mL barrel reads 0–30 units, 0.5 mL reads 0–50, 1 mL reads 0–100. U-40 is the trap: a different scale, 40 units per mL, so reading a U-100 dose off a U-40 barrel misdoses by 2.5×. That same 0.10 mL draw is 10 units on U-100 and 4 on U-40. Pick U-40 only if your syringe says so (mostly veterinary). Draws under about 5 units are hard to measure, so the calculator flags them and offers a water volume that spreads the dose wider.

How many units is a given dose?

No universal answer, because units depend on how concentrated your vial is — which depends on how much water you added. The same 250 mcg dose is 10 units if you used 2 mL, and 5 units if you used 1 mL. That's why a units-per-dose chart from a forum can be confidently wrong for your vial: it's answering for someone else's ratio.

Mixing two peptides in one syringe

This calculator does one compound at a time, so for a blend, run it once per peptide and add the draws — none of the top-ranking calculators does that for you. It matters because the popular blends are what people are measuring:Glow, Klow and theWolverine stack are multi-vial protocols combined into one shot. Whether two peptides should share a syringe is a different question from what the units are. For most pairs nobody has studied it, so ourmixing-compatibility reference defaults to "insufficient data" rather than a green light, andcan you mix peptides in one syringewalks the checks.

Why we handle IU honestly (and most calculators don't)

An International Unit measures biological potency, not mass, and the IU-to-mg factor differs for every molecule that has one. Human growth hormone (somatropin) is calibrated to the WHO international standard at 3 IU per mg, so 1 IU is about 0.333 mg. Most research peptides — BPC-157, TB-500, ipamorelin, the GLP-1s — have no defined IU at all and should be dosed in mg or mcg. This tool shows the IU option only when a compound has a real, cited factor, and prints the factor it used.

Frequently asked questions

How many units is 250 mcg on an insulin syringe?

It depends on your concentration. From a 5 mg vial mixed with 2 mL of bacteriostatic water, 250 mcg is 0.10 mL — 10 units on a U-100 syringe. Halve the water and it's 5 units.

Can I use sterile water instead of bacteriostatic water?

Not interchangeably, for a vial you'll enter more than once. Sterile Water for Injection "contains no bacteriostat, antimicrobial agent or added buffer and is supplied only in single-dose containers," per itsFDA label, which says to discard the unused portion. The benzyl alcohol is why a BAC-water vial serves dose after dose.

Does this work for tirzepatide or semaglutide?

Yes — the arithmetic is identical for any freeze-dried peptide you reconstitute. Pick a compound to auto-fill known vial sizes, or type your own. The "Tirzepatide-scale" example loads a 15 mg vial in 3 mL with a 2.5 mg dose: 0.50 mL, or 50 units.

Is this peptide calculator free?

Yes, and there's nothing to buy. No account, no email, no cart. We don't sell peptides or link to anyone who does — unusual for this search, where the top-ranking results are all storefronts, and the reason we can tell you when a number is folklore.

References

1. 1.Bacteriostatic Water for Injection, USP — FDA label (Hospira, Inc.)DailyMed / FDA
2. 2.Sterile Water for Injection, USP, Single-Dose Vial — FDA label (Hospira, Inc.)DailyMed / FDA
3. 3.General Chapter ⟨797⟩ Pharmaceutical Compounding — Sterile PreparationsUSP–NF · beyond-use dating
4. 4.Benzyl alcohol (phenylmethanol), CID 244PubChem / NIH

Reconstitution guidance current as of July 2026. Sources above were last checked 17 Jul 2026. Educational information, not medical advice.