Reconstituting a peptide vial means finding its concentration (peptide mass divided by the water added), then converting a target dose into a draw volume and a syringe reading: draw volume = dose / concentration, and syringe units = draw volume x units per mL of the syringe. This post is the arithmetic behind that sum. It does not tell you what to put in a vial or what dose to use, and neither does the reconstitution calculator it explains.
The vial label is a total, not a dose
A vial marked 5 mg holds 5 mg of dry powder. That figure is the total content. It says nothing about a dose, and it is the single most common place a calculation goes wrong: people read 5 mg and start thinking in doses, when the label is describing everything in the container.
Written as micrograms, 5 mg is 5000 mcg. Research peptides are usually discussed in micrograms and compounded GLP-1 products in milligrams, which is why a calculator that lets you switch units per field is more useful than one that assumes.
The water volume decides the concentration, nothing else
Once you add bacteriostatic water, the vial has a concentration:
concentration = peptide in the vial / bacteriostatic water addedFive milligrams in 2 mL is 2.5 mg per mL. The same 5 mg in 1 mL is 5 mg per mL, and in 5 mL it is 1 mg per mL. The amount of peptide never changed. What changed is how much liquid each microgram is spread through, and therefore how far up the barrel a given dose sits.
That is the entire reason the water volume matters arithmetically. A more dilute vial makes the same dose a bigger, easier to read draw. A more concentrated vial makes it a smaller one, which is harder to measure and where a half unit misread costs proportionally more.
How do you convert a vial concentration to a syringe reading?
Two more lines finish the job:
draw volume = target dose / concentration
syringe units = draw volume x units per mL of the syringeA U-100 insulin syringe is graduated at 100 units per millilitre. So with a 5 mg vial in 2 mL of water, at 2.5 mg per mL, a 250 mcg dose is 0.25 divided by 2.5, which is 0.1 mL, and 0.1 mL on a U-100 barrel reads as 10 units. If you hold a U-40 barrel instead, the same 0.1 mL reads as 4 units, which is covered in U-100 and U-40 syringes read the same volume differently.
Work the sum the other way if you want a sanity check: at 2.5 mg per mL, one U-100 unit carries 25 mcg, so ten units carry 250 mcg. Getting the same answer from both directions is a good habit, and it is exactly what the calculator shows when you open the formula panel.
How many doses a vial holds
Divide the vial by the dose. Five milligrams is 5000 mcg, so at 250 mcg per dose the vial holds 20 whole doses with nothing left over. At 300 mcg per dose it holds 16, and 200 mcg is stranded at the bottom, because the last part dose is not a dose. Calculators that report a decimal here are hiding that remainder; this one reports the whole doses and the leftover separately.
What this post deliberately does not say
It does not say how much water to add to your vial, because the right answer depends on the product in front of you and its own labelling, and no arithmetic can supply it. It does not say what dose to use. For compounds with an approved product label, such as semaglutide and tirzepatide, the labelled schedules are reproduced on their pages with the source named. For research compounds like BPC-157 and TB-500 no such label exists, so no range is published here at all.
Bacteriostatic water itself is a multiple dose preparation preserved with benzyl alcohol, according to the Pfizer labelling for Bacteriostatic Water for Injection USP. How long any particular reconstituted product remains usable is a question for that product’s own labelling, not for a calculator.
In short: concentration = peptide mass / water added, and draw volume = target dose / concentration, converted to syringe units by multiplying by the syringe’s units per mL.