Dose math tool
Bacteriostatic water calculator
Short answer
What it does: this calculator solves the reconstitution arithmetic backward. Instead of picking a water amount and seeing where a dose lands, it starts from the syringe mark and returns the bacteriostatic water amount that puts a stated dose on it.
The one fact it leans on: a U-100 insulin syringe marks 100 units per mL, so every unit is one hundredth of a millilitre of liquid.
What goes in: a vial size in milligrams, a dose to measure in milligrams, and a target mark. Nothing is prefilled, because the arithmetic only means something for the numbers actually typed, not for a value chosen in advance.
What comes out: the water amount in mL, the concentration that mix creates, and a check row confirming the dose reads as the chosen mark.
What it never does: suggest a dose, a concentration, or a water amount. It converts what is typed, and this page makes no network calls, so nothing typed goes anywhere.
Solve for the water amount
Bacteriostatic water calculator
Converts the numbers you type. It does not recommend a dose.
Arithmetic only. Check the result against your own vial, your own reconstitution, and your own syringe before you trust any unit count.
How the math works
The barrel fact: U-100 means 100 units per mL, so each unit is one hundredth of a millilitre. The 20-unit mark sits at 0.2mL and the 50-unit mark at 0.5mL, before any vial enters the picture.
What the dose fixes: for a dose to read as a chosen mark, that many milligrams has to occupy exactly the mark's volume, which pins one concentration: the dose divided by that volume.
What the vial fixes: with the concentration pinned, the water amount is whatever dilutes the whole vial down to it: vial milligrams divided by that concentration.
The check row: the dose divided by the concentration has to hand back the mark's volume. When it does, the mix is consistent, and any other dose reads off the same barrel by proportion.
Why this one runs backward: most mixing math starts from a water amount and reports where a dose lands. This tool starts from the mark, because the mark is the number the barrel actually shows.
| Step | The arithmetic | The result |
|---|---|---|
| The mark as a volume | 20 units x 0.01mL | 0.2mL |
| The concentration the mix needs | 2mg / 0.2mL | 10mg per mL |
| The water that reaches it | 10mg / 10mg per mL | 1mL |
| The check | 2mg / 10mg per mL | 0.2mL, the 20-unit mark |
| Vial size | Mark 10 | Mark 20 | Mark 25 | Mark 40 | Mark 50 |
|---|---|---|---|---|---|
| 5mg | 0.25mL | 0.5mL | 0.625mL | 1mL | 1.25mL |
| 10mg | 0.5mL | 1mL | 1.25mL | 2mL | 2.5mL |
| 20mg | 1mL | 2mL | 2.5mL | 4mL | 5mL |
| 30mg | 1.5mL | 3mL | 3.75mL | 6mL | 7.5mL |
One proportion to keep: the water amount moves in straight lines with all three inputs: double the vial or the mark and it doubles, double the dose and it halves.
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On this page
How do I make 2mg read as 20 units?
It scales with the vial: the water amount in mL comes out to one tenth of the vial size in milligrams, so a 10mg vial lands on 1mL, a 5mg vial on 0.5mL, and a 30mg vial on 3mL.
Where the tenth comes from: 20 units is 0.2mL on a U-100 barrel, and holding 2mg inside that volume means a concentration of 10mg per mL; the water amount is always the vial size divided by that concentration.
It scales with the mark too: at a 40-unit target every one of those water amounts doubles, because the mark's volume doubled while the dose stayed put.
How much bacteriostatic water do I add to a 10mg vial?
No single number exists: a 10mg vial takes 1mL to put 2mg on the 20-unit mark and 2.5mL to put the same 2mg on the 50-unit mark, and both are correct arithmetic for the same vial.
Why one-number answers mislead: any page that prints a single water amount for a vial size is quietly assuming a dose and a mark, and the assumptions are doing the work, not the vial.
The honest form of the question: given this vial, this dose to measure, and this mark, what water amount makes the three agree. That is exactly the solve the tool at the top runs.
What if the water amount comes out huge?
What a huge result means: past 10mL the mark stops being practical, because the water would not fit the vial it is meant to mix. A 30mg vial, a 0.5mg dose, and a 50-unit target would call for 30mL.
Why it blows up: the water amount grows in proportion to the vial and the mark, and shrinks in proportion to the dose, so a small dose against a big vial and a high mark inflates fast.
What moves it back: halving the target mark halves the water amount, and the tool flags any result over 10mL instead of printing a number that cannot be used.
Where this sits in the dose-math cluster
The cluster: this tool is one page of a set we built around a single rule: dose math is arithmetic on stated inputs, never advice.
- The dose-math hub: every tool and explainer in the set, in one place.
- Reconstitution calculator: the forward direction, starting from a water amount and reporting where a dose lands.
- Units to mg converter: converts between the barrel reading and milligrams once the concentration is known.
- Why the water amount changes the units: the explainer behind the proportion this tool uses.
- What a unit is on an insulin syringe: the barrel scale itself, before any vial math enters.
The disclaimer
Every page is reviewed by medical professionals before it ships, and written with longtime biohackers who were doing this before it was a trend. Reviewed still does not mean prescribed: nothing here is medical advice.
It is research, trial data, and reported use, with the numbers intact so you can check them. For decisions about your body, see a doctor who can look at your labs.
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