Dose-math tool
Reconstitution calculator: from target concentration to water
Short answer
The one formula: water to add, in mL, equals the milligrams in the vial divided by the concentration you want in mg per mL. The tool treats the water added as the final volume and does not model the powder's own displacement.
What the tool does: both directions of that division. A vial size and a target concentration return the water; a vial size and a water amount return the concentration, plus what one syringe unit carries.
What it never does: choose the concentration. Every field starts empty, because the target is a decision, not a constant we can ship.
The syringe side: a U-100 syringe marks 100 units per mL and a U-40 marks 40, so the per-unit answer moves with the syringe you select.
Where this sits: one of the converters listed on the peptide dose math index, next to the units-to-mg converter and the bac water calculator, which enters the same division from a syringe mark rather than a target concentration.
The calculator
Two ways in: from a target concentration, or from a water amount. The vial and the syringe are shared fields; the rest follows the mode.
Reconstitution calculator
Converts the numbers you type. It does not recommend a dose.
| Water added | Concentration | One unit carries |
|---|---|---|
| 0.5mL | ||
| 1mL | ||
| 2mL | ||
| 3mL | ||
| 5mL |
Arithmetic only. This is arithmetic on the numbers entered. It has no knowledge of any real vial, reconstitution, or syringe.
The privacy side: the arithmetic runs entirely in your browser; this page makes no network calls.
The next conversions: with a concentration in hand, the units-to-mg converter turns any milligram amount into a barrel reading, and the bac water calculator runs the water number on its own.
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On this page
How the math works
Concentration first: mg per mL equals vial milligrams divided by water mL. Every other number on this page is that one line rearranged.
Solving for water: flip the same line. Water mL equals vial milligrams divided by the target mg per mL, so a 10mg vial against a chosen target of 5mg per mL works out to 2mL of water.
The per-unit step: divide the concentration by the units on the syringe. At 5mg per mL on a U-100, one unit carries 0.05mg, or 50mcg; the same concentration on a U-40 puts 0.125mg on each unit.
| Step | Arithmetic | Result |
|---|---|---|
| water to add | 10mg ÷ 5mg per mL | 2mL |
| concentration, checked | 10mg ÷ 2mL | 5mg per mL |
| one unit | 5mg per mL ÷ 100 units per mL | 0.05mg (50mcg) |
| ten units | 0.05mg × 10 | 0.5mg |
Same vial, less water: the same 10mg vial with 1mL instead gives a concentration of 10mg per mL, with 0.1mg on every U-100 unit. The vial never changed; the water did.
What changes your units?
Only two things move what one unit carries: the concentration in the vial and the scale printed on the syringe.
The water lever: more water at the same vial size lowers the concentration and shrinks the per-unit number, and less water does the opposite. The full walkthrough of that tradeoff is in how the bac water volume changes your units.
The syringe lever: a U-40 unit is 0.025mL against 0.01mL on a U-100, so the same barrel mark holds 2.5 times the liquid. The selector in the tool exists for exactly this reason.
How much bacteriostatic water do I add?
However much turns the vial into the concentration you chose: vial milligrams divided by target mg per mL, with the answer in mL.
Our line: we do not pick that target, and the tool does not either. Any chart that names one water amount for every vial has quietly made the decision for you.
What bac water is: sterile water for injection carrying benzyl alcohol as a preservative, 0.9% or 1.1% on the Hospira label, packaged in multi-dose vials meant for repeated withdrawals.
Does more water mean a weaker dose?
No: the milligrams in the vial stay exactly the same, so a dose measured against the new concentration delivers the same amount. What shrinks is the milligrams on each unit, not the milligrams in the vial.
The real change: every barrel reading now means something different, and a units number memorized from the old mix is worth nothing at the new one.
Weaker per mark: that is the honest summary. The dose follows the math, and the math follows the water.
What happens if I add too much water?
The arithmetic still works, because the concentration simply falls. The limits are physical, a vial only holds so much liquid, and practical, a very dilute mix moves the same dose into a larger volume.
Where it bites: at 10mg in 5mL the concentration is 2mg per mL and one U-100 unit carries 0.02mg, so each unit of draw error moves about 1% of a 2mg dose.
The other extreme: too little water compresses the dose below the smallest mark the barrel prints. One unit on a U-100 scale is 0.01mL, and how finely a particular syringe is marked is a property of that syringe rather than of the scale.
After the mix: a reconstituted vial is on a clock, and how long a mixed peptide keeps covers what changes once the water goes in.
How this page is sourced
Mostly arithmetic: every number the tool returns is division or multiplication you can redo by hand, and the worked example is recomputed line by line before this page ships.
The part that is not arithmetic: the two barrel scales the tool offers. That a U-100 barrel carries 100 units in a milliliter and a U-40 carries 40 is a device and label fact, and division cannot produce it. Both are cited below, beside the bacteriostatic water definition.
What changed on 30 July 2026: a citation audit found the scale facts uncited, an unsupported claim that the powder's displacement is small, and a sentence treating every U-100 barrel as marked in whole units. The scales now carry two approved labels. The tool never modeled displacement, so the page now says only that, without a claim about its size. And the graduation sentence now names the smallest mark as a property of the individual syringe, because this site holds no document that specifies graduation intervals.
Three sources: three approved prescribing informations, all read on DailyMed.
The standard: the sourcing and citation-verification standard for the site lives on the methodology page, and corrections land there first.
Last reviewed: 28 July 2026.
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1
Hospira, Inc. Bacteriostatic Water for Injection, USP (prescribing information / package label). DailyMed, U.S. National Library of Medicine. DailyMed, current label.
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2
Humalog (insulin lispro) injection, for subcutaneous or intravenous use. Prescribing information, Eli Lilly and Company. Dosage Forms and Strengths, printing the strength as 100 units/mL (U-100); and the Principal Display Panel for the 10 mL vial, reading "Use only with a U-100 syringe". Retrieved 30 July 2026, HTTP 200. DailyMed SPL, set id c8ecbd7a-0e22-4fc7-a503-faa58c1b6f3f.
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3
Vetsulin (porcine insulin zinc suspension) injection. Label, Merck Sharp & Dohme Corp. Description and How Supplied, giving 40 IU/mL; the client information sheet, stating that U-40 insulin contains 40 units/mL; and Dosage and Administration, stating that use of a syringe other than a U-40 syringe will result in incorrect dosing. Retrieved 30 July 2026, HTTP 200. DailyMed SPL, set id 4472644f-edc1-4a4f-8506-888bd8ad895a.
The disclaimer
Every page is reviewed by medical professionals before it ships, and written with longtime biohackers who were doing this before it was a trend. Reviewed still does not mean prescribed: nothing here is medical advice.
It is research, trial data, and reported use, with the numbers intact so you can check them. For decisions about your body, see a doctor who can look at your labs.
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