How water volume changes the maths
The amount in a vial is fixed when it is filled. Water only decides how thinly that amount is spread. More water means a weaker liquid and more units for the same amount; less water means a stronger liquid and fewer units.
What changes and what does not
| Add more water | Add less water | |
|---|---|---|
| Amount in the vial (mg) | Unchanged | Unchanged |
| Concentration (mg/mL) | Lower | Higher |
| Units needed for a given amount | More | Fewer |
| Amount held by one tick | Less | More |
A 10 mg vial at four dilutions
| Water added | Whole vial | Concentration | 1 unit holds | 10 units holds | Units that hold 1 mg |
|---|---|---|---|---|---|
| 1 mL | 100 units | 10 mg/mL | 0.1 mg | 1 mg | 10 units |
| 2 mL | 200 units | 5 mg/mL | 0.05 mg | 0.5 mg | 20 units |
| 3 mL | 300 units | 3.333 mg/mL | 0.03333 mg | 0.3333 mg | 30 units |
| 5 mL | 500 units | 2 mg/mL | 0.02 mg | 0.2 mg | 50 units |
Read across the 2 mL row: 10 mg in 2 mL is 5 mg/mL, the whole vial is 200 units, each unit holds 0.05 mg and each 10 units holds 0.5 mg. Go to 5 mL and every unit holds less than half as much, so the same amount takes two and a half times as many units.
A 5 mg vial at the same dilutions
| Water added | Whole vial | Concentration | 1 unit holds | 10 units holds | Units that hold 1 mg |
|---|---|---|---|---|---|
| 1 mL | 100 units | 5 mg/mL | 0.05 mg | 0.5 mg | 20 units |
| 2 mL | 200 units | 2.5 mg/mL | 0.025 mg | 0.25 mg | 40 units |
| 3 mL | 300 units | 1.667 mg/mL | 0.01667 mg | 0.1667 mg | 60 units |
| 5 mL | 500 units | 1 mg/mL | 0.01 mg | 0.1 mg | 100 units |
Notice that a 5 mg vial with 1 mL and a 10 mg vial with 2 mL are both 5 mg/mL. Different vials can reach the same concentration, and once the concentration matches, every syringe reading means the same amount. That is why the conversion pages are organised by concentration.
The trade-off is readability
Water volume is a question of how the amount maps onto the syringe scale.
- With more water, a given amount spans more ticks, so a small amount is easier to read. The limit is the barrel: a result above 100 units will not fit in one 1 mL syringe, and above 30 or 50 units will not fit the smaller barrels.
- With less water, a given amount spans fewer ticks. Each tick is worth more, so being one tick out matters more, and a very small amount can fall between lines.
For example, at 10 mg/mL an amount of 0.25 mg is 2.5 units, which sits between marks on most barrels. At 2.5 mg/mL the same 0.25 mg is 10 units, which lands on a printed number.
The volume to add is not something to choose from a table. It is written on your pharmacy label or the product instructions, and that is the figure to use. If you want to see what a different volume would do to the numbers before asking your pharmacist about it, the bacteriostatic water calculator works backwards from a target concentration.
Enter what actually went in
The calculation uses the water that is in the vial, not the water that was planned. If a different amount went in, the concentration is different and the original figures no longer apply. Do not adjust by guesswork; tell your pharmacist what happened.
One assumption to know about
The formula treats the final volume as equal to the water added. The dry powder takes up a small amount of space as well, so the true volume is slightly higher. Where a manufacturer's label states the concentration after mixing, for example "after reconstitution each mL contains 5 mg", use the label's figure instead of working it out.
Blends
A vial with two substances follows the same rule for each one separately. A vial holding 5 mg of one and 5 mg of another in 2 mL is 2.5 mg/mL of each. Drawing 10 units takes 0.25 mg of both; the two cannot be drawn independently. The blend calculator handles this.
Questions
Does adding more bacteriostatic water make a peptide weaker?
It lowers the concentration, so each unit of liquid holds less. The total amount in the vial does not change. A given amount simply takes more units to draw.
How do I calculate concentration after adding water?
Divide the amount in the vial by the volume of water added. A 10 mg vial with 2 mL is 10 ÷ 2 = 5 mg/mL. With 3 mL it is 3.333 mg/mL.
How much water should I add to a 10 mg vial?
Use the volume on your pharmacy label or product instructions. This site does not recommend one. Any volume can be calculated: 1 mL gives 10 mg/mL, 2 mL gives 5 mg/mL, 3 mL gives 3.333 mg/mL and 5 mL gives 2 mg/mL.
What if I added a different amount of water than the instructions said?
The concentration is different from the one your instructions assumed, so the original unit figures no longer apply. Contact your pharmacist and tell them the amount that went in.