Quick answer: adding more bacteriostatic (BAC) water doesn't change how many mg are in the vial. It lowers the concentration, so the same mg takes more units on the syringe. If you add twice the water, the same amount takes twice the units. What changes is the unit count, not the amount of drug, which is why a unit number only means something alongside the concentration of the vial it came from.
The formula: U-100 units per mg = (mL of water added × 100) ÷ mg in the vial.
Educational only, not medical advice. This article explains arithmetic. It doesn't recommend a dose, a volume of water or any compound. How much diluent to add, and the amount you're meant to draw, come from your prescriber, your pharmacist or the product's instructions.
Why more water means more units for the same mg
Think of the vial as a fixed amount of drug. The water you add sets how thinly that amount is spread:
- Concentration (mg/mL) = mg in the vial ÷ mL of water added
- mL to draw = the amount in mg ÷ concentration
- Units on a U-100 syringe = mL × 100
Add more water and step 1 gives a smaller concentration. A smaller concentration makes step 2 give a larger volume, and a larger volume is more units. The mg you end up with in the syringe is the same; it just arrives in more liquid.
Our reconstitution pillar guide walks through the whole process, and the calculator below does the three steps for you.
Reconstitution calculator
Enter the numbers from your own vial, and pick the syringe in your hand. Check the U- number and size printed on the syringe box.
Concentration
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Draw volume
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Syringe units
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U-100 1 mL, holds 100
Draws in the vial
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Math only, not dosing advice. Follow your prescriber or label. Real vials give slightly less than the full-draw count: a little liquid stays in the vial and needle. Your last setup is saved in this browser only; Clear removes it.
Save your vial in Caliper and it remembers the concentration and use-by date you entered, alongside your dose log. The calculator above is a browser tool and is not part of the app.
Save my vial in CaliperThe same vial with different amounts of water
Example math only, with a hypothetical 10 mg vial. It isn't a recommendation of any volume or amount; it shows the rate at which units scale.
| Water added | Concentration | mL per 1 mg | U-100 units per 1 mg |
|---|---|---|---|
| 1 mL | 10 mg/mL | 0.1 mL | 10 units |
| 2 mL | 5 mg/mL | 0.2 mL | 20 units |
| 3 mL | about 3.33 mg/mL | 0.3 mL | 30 units |
| 5 mL | 2 mg/mL | 0.5 mL | 50 units |
Two patterns to notice:
- Units scale in step with the water. For a 10 mg vial, units per mg is simply mL added × 10.
- The total never moves. Every row still holds 10 mg. More water doesn't stretch the vial further; the number of draws depends on the total mg and the amount per draw.
For a side-by-side look at the trade-offs (how easy each volume is to measure, whether a draw fits the syringe, whether the liquid fits the vial), see how much BAC water to add to a 10 mg vial.
So does more water make it weaker?
Each unit is weaker, because it holds less drug. The vial isn't weaker, and an amount measured in mg is the same amount whichever volume you mixed with. "Weaker" only becomes a problem if you keep drawing the same unit count you used with a more concentrated vial. Math only: in the table above, 20 units is 2 mg at 1 mL of water but only 0.4 mg at 5 mL.
That's the mistake behind many of the questions people ask online: the unit count was carried over while the water changed.
When the unit count has to change
Recalculate before your next draw whenever:
- You mixed a new vial with a different volume than last time.
- The vial amount changed, even if the water didn't. A 15 mg vial with the same water as a 10 mg vial is 1.5 times as concentrated.
- Someone else's unit count is in front of you. Units from a friend, a forum or a video only fit their vial and their water. Your number comes from your concentration and the amount your prescriber set.
- Your pharmacy changed the concentration of a vial that came already mixed. See recalculating units for a new vial.
Can you add more water to a vial you've already mixed?
Only if your pharmacist or the product instructions say so. Adding liquid to a mixed vial changes the concentration of whatever is left, and working out that new concentration needs you to know exactly how much drug remains, which is hard after several draws. It also adds another puncture to a multi-dose vial. If you think a vial was mixed with the wrong volume, ask your pharmacist how to handle it rather than adjusting it yourself.
Water added vs final volume
Most calculators, including ours, use the water you added as the volume. Some powders take up a little space once dissolved, so the final volume can be slightly more than the water added. For everyday arithmetic the difference is small, but it's one reason two calculators can disagree slightly. If your pharmacy gives you a concentration on the label, use that number.
A quick self-check
Before you draw from a vial mixed with a different volume:
- Write the new concentration on the vial (mg ÷ mL).
- Convert the amount you've been prescribed in mg to mL, then to units.
- Check it backwards: units ÷ 100 × concentration should give back the mg.
- Check the direction: more water → more units; less water → fewer units. If your answer moved the other way, recheck it.
- Make sure the draw fits your syringe. Our guide to reading insulin syringe tick marks helps when a number lands between lines.
Tracking it in Caliper
The reconstitution calculator on this site takes your vial strength, how much BAC water you added and the amount you've been prescribed, and shows the result in units and mL in your browser. Save the vial in Caliper and it gets its own recorded concentration, so a new vial mixed with a different volume doesn't inherit the old numbers. Caliper records what you enter; it doesn't recommend or calculate a dose or a volume of water.
FAQ
If I add more BAC water, do I draw more units for the same mg?
Yes. More water lowers the concentration, so the same mg takes more volume and therefore more units. Double the water and the same mg takes double the units.
Does adding more BAC water change the amount of drug in the vial?
No. The mg in the vial stays the same. Water only changes the concentration, which changes how many units each mg takes.
Does more BAC water make a peptide weaker?
Each unit holds less, so a unit is weaker, but the vial and an amount measured in mg are not. Problems only start if you keep drawing the old unit count after changing the water.
How do I work out units per mg?
On a U-100 syringe, units per mg = (mL of water added × 100) ÷ mg in the vial. For a 10 mg vial, that's the mL added × 10.
Does more water make the vial last longer?
No. The number of draws depends on the total mg and the amount per draw. The vial's use-by date still applies.
Can I add more water to a vial that's already mixed?
Only if your pharmacist or product instructions say so. It changes the concentration of what's left, which is hard to calculate after several draws, and it adds another puncture.
The takeaway
More water, same mg, more units. The water you add sets the concentration, and the concentration turns your prescribed mg into a unit count. Change the water and the unit count has to change with it; the amount of drug doesn't. Write the concentration on every vial, recalculate when anything changes, and ask your pharmacist when the paperwork doesn't add up.
Sources
- DailyMed, Bacteriostatic Water for Injection, USP label
- FDA, Dosing errors associated with compounded injectable semaglutide products
Make the math (and the record-keeping) automatic
Caliper is a private, local-first tracker for GLP-1 and peptide protocols. You record the amount you took and it logs each dose with one tap, reminds you when the next one is due, tracks vials with reconstitution and use-by dates, and keeps injection sites, side effects and weight trends on one timeline. Your health data stays on your device, and Caliper never recommends or calculates a dose; it organizes what you and your provider decide.
Start tracking your vials and doses with Caliper →
For adults 18+.
Medical disclaimer: This article is for general educational purposes only and is not medical advice. It does not recommend any dose, dilution, compound, product, supplier or protocol. Always follow your prescriber's instructions and the product label, and talk to a pharmacist or healthcare provider about any question regarding your medication, its preparation, storage, measurement or administration.