Bacteriostatic water vials come in three labeled volumes: 3 mL, 10 mL and 30 mL. Each vial actually contains a little more than its label, typically a few tenths of a millilitre, so that the full labeled volume can be withdrawn after allowing for liquid that clings to the vial and the withdrawal device.
Why 30 mL is the largest
The pharmacopeial standard for bacteriostatic water limits the container to 30 mL, because the preservative is validated for a limited number of entries over a limited period. There is no 50 or 100 mL bacteriostatic water; larger single volumes are supplied as preservative-free sterile water. See why the 30 mL limit.
Plan on the labeled volume
The overfill is a guarantee, not a bonus. A 30 mL vial should be expected to yield 30 mL, not 31. Cost-per-millilitre comparisons use the labeled volume. Details are in fill volume and overfill.
Which size
The one a bench will use within 28 days of opening: roughly 3 mL for under 0.1 mL a day, 10 mL for up to about 0.35 mL a day, 30 mL above that. See choosing a vial size or use the calculators.
All BAC Water Supply products are supplied for laboratory research use only and are not for human or veterinary use.