How many doses can I get out of this vial? The arithmetic is a single division, and the useful answer is always smaller than the division suggests. This post does the division, then explains the gap between the theoretical number and what you actually get.
The calculation
doses per vial = vial mass / amount per doseThat is all it is. A 5 mg vial at 0.25 mg per dose gives exactly twenty. The reconstitution calculator on this site reports this as a result, and also reports how much is left in the vial after a draw, which is the same number approached from the other direction.
A fractional result is not an error. A 5 mg vial at 0.3 mg per dose gives 16.67, which means sixteen full doses and a final partial one, and the remaining 0.05 mg is left over.
Why the real number is lower
The division assumes every milligram in the vial reaches a syringe. Several things prevent that, and they compound:
Filter and needle retention. Material adsorbs to surfaces it passes through. The loss is small per pass but it applies to every dose.
Dead volume. A syringe retains solution that never leaves the barrel when the plunger is fully depressed. This is a fixed amount per syringe, so it hurts small draws proportionally much harder. At a 0.1 mL draw on a syringe with 0.02 mL of dead volume, roughly a fifth of the drawn liquid never gets delivered, and at a 0.02 mL draw the same dead volume is most of it.
Material left in the vial. Solution clings to the wall, and the last draws draw from a residue rather than from a full volume.
Reconstitution losses. Some peptide does not enter solution, which lowers the effective concentration below what the arithmetic says.
None of these are predictable from the label, which is why calculators report a theoretical count and say so.
How to make the gap smaller
Use the largest draw volume your syringe measures accurately. Dead volume is fixed, so a 1 mL draw loses a smaller fraction than a 0.05 mL draw of the same volume twice. Reconstitute to a concentration that lands whole draws on the barrel, which the BAC-water solver will work out for you, and draw from a vial that has been given time to dissolve completely rather than immediately after adding water.
Where doses per vial matters beyond counting
Cost. Cost per dose is the vial price divided by this number, which is why the cost calculator reports both. A vial that yields fewer doses has a higher real cost per dose than the theoretical figure suggests.
Inventory. Days remaining is this number divided by your frequency, and it inherits the same optimism.
Whether the vial is the right size. A 5 mg vial at 0.25 mg gives twenty doses, which is a lot of material that has to stay in solution for that long. If you need four, a 2 mg vial is a better match, and the arithmetic that tells you so is the same division.
What this is not
Doses per vial is arithmetic on a mass and a number. It does not recommend a dose, a frequency, a vial size or a product, and the examples above use invented figures to show the arithmetic rather than to describe any real protocol.
Related: how to reconstitute a peptide vial and how peptide cost per dose calculations work.