How much bacteriostatic water do I add to a 5 mg vial?

Any volume the vial holds. It changes the concentration and therefore the units you draw, and it changes nothing about the dose you receive or how many doses the vial contains. Choose the volume that puts your dose on a printed mark.

Does more water mean a weaker dose?

No. More water means a weaker solution, so you draw a larger volume to deliver the same dose. The peptide in the vial is fixed and the number of doses is unchanged.

Is 1 mL or 2 mL better?

Neither is better in general. 2 mL halves the micrograms carried by each unit compared with 1 mL, which means finer control and a larger draw. If your dose lands on a printed line at 2 mL and between two lines at 1 mL, 2 mL is better for you.

The limit is what the vial physically holds, which for a small vial is usually somewhere between 2 and 5 mL including headroom. Beyond that it will not fit, and there is no arithmetic reason to go further.

How Much Bacteriostatic Water to Add

There is no correct amount, and that is not a dodge. The volume changes the number you read on the syringe and nothing else at all.

What the volume does and does not change

It does not change how much peptide is in the vial, because that was fixed when it was manufactured. It does not change how much peptide is in a dose, because you compensate by drawing a different volume. It does not change how many doses the vial holds.

What it changes is how many micrograms one printed unit on the syringe carries, and therefore whether your dose lands on a line you can actually read. That is the entire decision.

The same vial and dose at five volumes

A 5 mg vial with a 250 mcg dose, on a 0.5 mL (50 unit) barrel:

Diluent	Strength	Per unit	Draw	Doses
1 mL	5 mg/mL	50 mcg	5 units	20
1.5 mL	3.33 mg/mL	33.3 mcg	7.5 units	20
2 mL	2.5 mg/mL	25 mcg	10 units	20
3 mL	1.67 mg/mL	16.7 mcg	15 units	20
5 mL	1 mg/mL	10 mcg	25 units	20

The doses column is identical all the way down, which is the point. The draw column is not. Highlighted rows put the dose exactly on a printed line.

How to choose

Work backwards. Decide the number you want to draw, pick something easy to read like 10, 20 or 25 units, and add the volume of diluent that puts your dose there. The diluent volume calculator does that division for you.

Two practical limits sit either side of it. Too little diluent and each unit carries so much that no dose between two marks exists, which the calculator flags. Too much and the vial will not physically hold it. Between those, it is a free choice.

The matched-volume arrangement, as many millilitres as the vial has milligrams, is the one worth knowing because it makes every unit exactly 10 mcg and removes the arithmetic. The reconstitution chart shows it as the diagonal.

Questions

How much bacteriostatic water do I add to a 5 mg vial?: Any volume the vial holds. It changes the concentration and therefore the units you draw, and it changes nothing about the dose you receive or how many doses the vial contains. Choose the volume that puts your dose on a printed mark.
Does more water mean a weaker dose?: No. More water means a weaker solution, so you draw a larger volume to deliver the same dose. The peptide in the vial is fixed and the number of doses is unchanged.
Is 1 mL or 2 mL better?: Neither is better in general. 2 mL halves the micrograms carried by each unit compared with 1 mL, which means finer control and a larger draw. If your dose lands on a printed line at 2 mL and between two lines at 1 mL, 2 mL is better for you.
Can I add too much?: The limit is what the vial physically holds, which for a small vial is usually somewhere between 2 and 5 mL including headroom. Beyond that it will not fit, and there is no arithmetic reason to go further.