Does adding more bacteriostatic water change my dose?

Bacteriostatic water does not change the amount of peptide in the vial, only the volume it is dissolved in. A 5mg vial holds 5mg whether you add 1ml or 3ml. Adding more water makes each dose a larger, easier-to-measure volume on the syringe; adding less makes the same dose a smaller volume. The calculator picks a water volume that lands your dose on a readable mark rather than between two of them.

Why is the number on my syringe different from the one on my vial?

A U-100 insulin syringe is marked in units, where 100 units is 1ml. So 10 units is 0.1ml and 50 units is half a millilitre. The number written on your vial is a mass in milligrams, not a volume, which is why it cannot be read off the syringe directly. Converting between them requires knowing how much water was added, which is the calculation this page performs.

How long does a reconstituted vial last?

Once bacteriostatic water is added, the benzyl alcohol in it limits how long the solution stays usable, and the common working figure is 28 days refrigerated. That matters for planning because a vial you cannot finish inside that window is partly wasted. The calculator shows how many doses a vial yields and how many days that takes, so a mismatch is visible before you reconstitute rather than after.

Does this tell me what dose to take?

No. This page performs arithmetic on numbers you type in. It does not suggest a dose, a compound, or a schedule, and the dose field starts from a neutral example rather than a recommendation. What dose is appropriate is a question for a clinician, not for a calculator.

Reconstitution calculator

Bacteriostatic water in, units on the syringe out.

Vial sizemgmg in the vialYour dosemcgthe dose you already takeDoses per weekhow oftenVial costoptional, to price the wasteDiscard afterdaysUSP default for an opened vial

The plan

Add 1ml of bacteriostatic water, then draw 5 units.

That gives 5mg/ml, and 5 units on a U-100 syringe is 250mcg.

Doses in the vial: 20
Lasts: 47 days
Wasted at discard: 8 doses
Cost per used dose: —

This vial does not get finished

At 3 doses a week you use 12 of 20 doses before the 28-day discard date. Add a vial cost above to see what that is worth in money.

A smaller water volume yields fewer doses per vial and wastes less. The table below shows the tradeoff.

Every dilution, compared

Ranked by whether the draw is readable first, then by waste.

Water	Draw	Doses	Lasts	Wasted
1ml	5u	20	47d	8
2ml	10u	20	47d	8
3ml	15u	20	47d	8
4ml	20u	20	47d	8
5ml	25u	20	47d	8
1.5ml	7.5u	20	47d	8
2.5ml	12.5u	20	47d	8
3.5ml	17.5u	20	47d	8

Keep the plan

Gireta tracks the vial from here: what is left in it, when it hits the discard date, and what the protocol is costing you a month.

Save this vial

How the calculation works

Reconstitution converts a mass into a volume. The vial holds a fixed amount of peptide measured in milligrams, and bacteriostatic water dissolves it into a liquid measured in millilitres. Dividing the mass by the water volume gives a concentration, and dividing your dose by that concentration gives the volume to draw. A U-100 syringe is marked so that 100 units is one millilitre, so that volume becomes a mark you can actually see.

The only free choice in that chain is how much water to add, and it does not affect how much peptide you receive. It affects how easy the dose is to measure. Too little water and a dose lands on two or three units, where a small misread is a large proportional error. Too much and the vial may not be finishable inside the window the solution stays good for. The recommendation above picks a volume that puts your dose on a whole or half unit where one exists.

Why the discard window matters before you mix

A reconstituted vial has a usable life, commonly taken as 28 days refrigerated, because the benzyl alcohol in bacteriostatic water is what keeps it stable and it does not last indefinitely. A vial that yields more doses than you will take inside that window is partly wasted, and that waste is decided at the moment you add the water rather than discovered weeks later. Working out doses per vial and days per vial before mixing is the entire reason this calculation is worth doing in advance.

Questions

Does adding more bacteriostatic water change my dose?: Bacteriostatic water does not change the amount of peptide in the vial, only the volume it is dissolved in. A 5mg vial holds 5mg whether you add 1ml or 3ml. Adding more water makes each dose a larger, easier-to-measure volume on the syringe; adding less makes the same dose a smaller volume. The calculator picks a water volume that lands your dose on a readable mark rather than between two of them.
Why is the number on my syringe different from the one on my vial?: A U-100 insulin syringe is marked in units, where 100 units is 1ml. So 10 units is 0.1ml and 50 units is half a millilitre. The number written on your vial is a mass in milligrams, not a volume, which is why it cannot be read off the syringe directly. Converting between them requires knowing how much water was added, which is the calculation this page performs.
How long does a reconstituted vial last?: Once bacteriostatic water is added, the benzyl alcohol in it limits how long the solution stays usable, and the common working figure is 28 days refrigerated. That matters for planning because a vial you cannot finish inside that window is partly wasted. The calculator shows how many doses a vial yields and how many days that takes, so a mismatch is visible before you reconstitute rather than after.
Does this tell me what dose to take?: No. This page performs arithmetic on numbers you type in. It does not suggest a dose, a compound, or a schedule, and the dose field starts from a neutral example rather than a recommendation. What dose is appropriate is a question for a clinician, not for a calculator.

This page calculates. It does not prescribe, and nothing here is a recommendation to take anything. Compound reference.