Bacteriostatic water calculator
Most calculators ask how much water you added. This one works the other way: how much should you add so the dose lands where you can read it.
Pick the draw you want, read off the water
| Vial | Your dose | for 10 units | for 20 units | for 25 units | for 40 units | for 50 units |
|---|---|---|---|---|---|---|
| 10 mg | 2 mg | 0.5 mL | 1 mL | 1.25 mL | 2 mL | 2.5 mL |
| 10 mg | 0.5 mg | 2 mL | 4 mL | 5 mL | 8 mL | 10 mL |
| 5 mg | 250 mcg | 2 mL | 4 mL | 5 mL | 8 mL | 10 mL |
| 5 mg | 500 mcg | 1 mL | 2 mL | 2.5 mL | 4 mL | 5 mL |
| 30 mg | 2.5 mg | 1.2 mL | 2.4 mL | 3 mL | 4.8 mL | 6 mL |
Read across to the draw size you want, and that cell is how much bacteriostatic water to add.
The rule behind the table
water (mL) = target units x vial (mg) / (100 x dose in mg)
The reason to care is readability. A dose that lands at 3 units sits between two marks you cannot reliably distinguish, and being one mark out is a 33 percent error. The same dose at 30 units is easy, and one mark out is 3 percent. Nothing about the drug changes, only your ability to measure it.
Or work it from your own numbers
25 printable pages: dosing cards for the twelve most-used compounds at three water volumes, titration planners, a vial log, and five pages on what to do when it has already gone wrong.
How much bacteriostatic water should I add?
There is no single correct amount. The water does not change the dose, only how far along the barrel that dose lands. More water means a bigger, easier-to-read draw of a weaker solution. Less water means a small, concentrated draw that is harder to measure precisely.
The practical rule is to pick a volume that puts your usual dose somewhere between roughly 10 and 50 units, because that part of the barrel is easy to read and small errors matter least. If the calculator tells you to draw 3 units, add more water. If it tells you to draw 90 units, add less.
Whatever you choose, write it on the vial. The number is meaningless a week later if you cannot remember how much water went in, and that is the single most common reason people end up re-dosing wrong.
Common questions
- How much bacteriostatic water do I add to a peptide vial?
- Any amount the vial holds. The water does not change your dose, only the volume it arrives in. Pick the amount that puts your usual dose between roughly 10 and 50 units on the barrel.
- Does more bacteriostatic water mean a weaker dose?
- No. The vial contains a fixed amount of peptide. More water spreads it through more liquid, so you draw a larger volume for the same dose. The amount of peptide you inject is unchanged.
- Can I add more water later?
- Adding water to an already reconstituted vial changes the concentration of everything left in it, so every number you worked out before becomes wrong. If you do it, recalculate from the new total volume.
- What if I added the wrong amount?
- Nothing is ruined. Work out the real concentration from what you actually added and use that. The peptide in the vial has not changed.
Common conversions
The same three-step maths, pre-filled for the doses people ask about most. Units on a U-100 insulin syringe, where 100 units is 1 mL.
- mg to units calculator Any dose at any concentration, both directions, with the full reference tables
- Units to mg calculator What the units you drew are in milligrams
- 1 mg to units The most-asked conversion, at every common setup
Related calculators and charts
Tools
- Peptide calculator The general tool, any compound
- Reconstitution calculator How much water to add, and why it matters
- Bacteriostatic water calculator (this page)
- How to read an insulin syringe U-100 vs U-40, and the 2.5x error
- The Protocol Pack Free 25-page printable reference
- mg to units calculator Both directions, any concentration, with the reference tables
- 1 mg to units on an insulin syringe The conversion table for every common concentration
- Units to mg calculator The mark you drew, back into milligrams
- Methodology Every formula, assumption and rounding rule
- About Peptulator Who publishes it, independence and contact
- Embed the calculator Free widget for blogs, clinics and newsletters, credit link only
Every one of the 27 compound calculators, charts and tools: the full list.