Home › Guides › Bac water vs. sterile water (and which to buy)
Bac water vs. sterile water (and which to buy)
Bac water, sterile water and saline are not interchangeable. Which one your vial needs, how much to add, and why an opened bottle has a clock on it.
Last reviewed · written by PepPrep · not medical advice
"Bac water" is just the short name for bacteriostatic water. They are the same product, and the short form is what most people actually say and type. You will see both on labels, so it's worth knowing that neither one is a different or lesser grade.
Both are sterile. The difference is one ingredient:
- Bacteriostatic water (bac water) is sterile water plus 0.9% benzyl alcohol. That trace of alcohol stops bacteria from growing after the vial's seal is first punctured, which is why an opened vial is generally considered usable for about 28 days. If you'll draw from the same vial multiple times, this is the one you want.
- Sterile water for injection has no preservative at all. It's perfectly clean until the moment you open it — and then the clock runs out fast. It's meant for mix-it-and-use-it-once situations, or for people avoiding benzyl alcohol.
- Sterile saline (0.9% sodium chloride) is sterile salt water matching your body's own salt level. Some product instructions specifically call for it; if yours do, use it.
| Water | Preservative | After opening | Buy it when… |
|---|---|---|---|
| Bacteriostatic | 0.9% benzyl alcohol | ~28 days | You'll draw from the vial more than once (most people) |
| Sterile water | None | Use immediately | Single-use mixing, or avoiding benzyl alcohol |
| Saline 0.9% | None (salt only) | Use immediately | The instructions specifically say saline |
Rule of thumb: multi-dose vial → bac water. Single-use → sterile water. Instructions say saline → saline.
"How much bac water do I add?"
This is the most common question about bac water, and the answer surprises people: there is no single correct amount. The volume you add is a choice, not a fixed property of what's in the vial. Adding more water doesn't change how much substance is in there — it only spreads that same amount across a larger volume, so each syringe marking is worth less.
What the choice actually controls is how easy the measurement is to read. Too little water and your measurement lands on a sliver of a marking; too much and you're injecting more liquid than you need to. The usual aim is a number that falls on a clean, easy-to-read part of the syringe.
Two things decide it for you:
- What your product's own instructions say. If they specify a volume, use that — it overrides every rule of thumb, including this page.
- What lands on a readable marking. Our reconstitution calculator works this out: put in the vial size, the volume you're considering, and the amount you want per draw, and it shows you where that falls on a U-100 syringe. Try a couple of volumes and keep whichever gives you the tidiest number.
What this page will not do is tell you how much to take. That's between you and whoever prescribed or supplied it. The calculator converts an amount you have already decided on into syringe markings — it's arithmetic, not advice.
Bottle sizes, and which to buy
Bac water is usually sold in 10 mL, 30 mL and occasionally 3 mL bottles.
- 10 mL is the common choice and the one most listings mean by default. It suits people mixing occasionally.
- 30 mL costs less per millilitre, but the 28-day clock starts when you first puncture it, the same as any other size. A big bottle you can't finish inside a month isn't a saving.
- 3 mL is worth knowing about if you mix rarely — a small bottle you finish beats a large one you throw away.
Buy the size you'll realistically use within about a month of opening, not the best price per millilitre.
Supplies mentioned: Mixing & Water