Key takeaways
- Concentration = amount in the vial ÷ water added.
- On a U-100 syringe, 100 units is 1 mL.
- If the draw falls between two markings on the syringe, you cannot measure it, and the sensible fix is a different water volume.
- Check every input against the label and your prescriber’s instructions.
What this tool does
A powder vial holds a fixed amount of compound. You add a measured volume of liquid, and the result has a concentration. The tool takes the vial amount, the water you added and the dose you want, and gives you the concentration, the volume to draw and the marking on a U-100 syringe.
It is a unit converter. It does not know whether your compound, your dose or your water volume is right.
How the arithmetic works
- Concentration (mg per mL) = vial amount (mg) ÷ water added (mL).
- Volume to draw (mL) = dose (mg) ÷ concentration (mg per mL).
- Syringe units = volume (mL) × 100.
Doses in micrograms work the same way once you convert: 1 mg = 1,000 mcg. A mix-up between mg and mcg is a 1,000-fold error, so write the unit next to every number.
A worked example
Example. These numbers are made up to show the steps. They are not a label value and not a suggestion.
A made-up vial of 10 mg, with 2 mL of water added.
- Concentration = 10 ÷ 2 = 5 mg/mL.
- A wanted 2.5 mg: 2.5 ÷ 5 = 0.5 mL.
- 0.5 mL × 100 = 50 units.
With 1 mL of water instead, the concentration is 10 mg/mL and the same dose is 0.25 mL, or 25 units. Same dose, different marking. That is why the water volume must be written on the vial.
What to check against your label
- The amount in the vial as printed on the label, and the unit.
- The diluent the instructions call for and how much. [verify against the product instructions]
- The syringe size and scale. A U-100 syringe is assumed. A different syringe changes the units.
- How long a mixed vial can be kept and where. [verify against the product instructions]
Frequently asked questions
Why does the tool warn when the draw is between markings?
A syringe can only be read at its markings. A draw that lands between two of them means you would be guessing the last part of the dose.
Can I use less water?
You can, and the sums change. A smaller volume means a stronger mix and a smaller draw, which is harder to read.
Does the tool say how much to take?
No. You give it the dose.
What about U-40 or other syringes?
The sums above assume U-100. For any other scale, convert from millilitres.
How Reko does this in the app
Reko has a reconstitution calculator that follows these steps, and it warns you when the draw falls between two syringe markings. You can attach a vial to your record, and Reko works out what is left in it from the doses you log. See the vial tracker and the calculator.
Reko records what you and your prescriber decide. It never suggests a dose, and this page is not medical advice.
- Product instructions for the compound you are mixing. [link]
- Syringe manufacturer's scale description. [link]