Guide
How to reconstitute a vial: the math.
Reconstitution means adding liquid to a powder in a vial. This guide covers only the arithmetic that follows: the concentration, the volume and the units on the syringe.
Vialist does arithmetic on numbers you give it. It does not give dosing advice. Every example here is for illustration only, so use the dose your prescriber gave you and the instructions on your product label or from your pharmacist.
What to mix with, how much to add and how to do it safely come from the instructions for your product, your prescriber or your pharmacist. This page does not replace them. It shows how to check the numbers once you know what went into the vial.
Step 1: find the concentration
The concentration is how much of the compound is in each mL of liquid.
concentration = amount in the vial ÷ water added (mL)
Step 2: find the volume for your dose
Use the dose your prescriber gave you, in the same unit as the vial (mg with mg, mcg with mcg, IU with IU).
volume in mL = dose ÷ concentration
Step 3: turn mL into units
A U-100 insulin syringe has 100 units in 1 mL, so each unit is 0.01 mL.
units on a U-100 syringe = volume in mL × 100
Worked examples
All three are examples only. Use the dose your prescriber gave you.
In mg. A 5 mg vial with 2 mL of liquid is 5 ÷ 2 = 2.5 mg per mL. An example amount of 0.25 mg is 0.25 ÷ 2.5 = 0.1 mL, and 0.1 × 100 = 10 units.
In mcg. A 10 mg vial with 2 mL of liquid is 10 ÷ 2 = 5 mg per mL. An example amount of 250 mcg is 0.25 mg, because 1000 mcg is 1 mg. Then 0.25 ÷ 5 = 0.05 mL, and 0.05 × 100 = 5 units.
In IU. A 5,000 IU vial with 10 mL of liquid is 5,000 ÷ 10 = 500 IU per mL. An example amount of 250 IU is 250 ÷ 500 = 0.5 mL, and 0.5 × 100 = 50 units. IU is a measure of activity, not mass, so it stays in IU and is never converted to mg.
More water, lower concentration
The same vial gives a different concentration for every amount of water, and so a different number of units for the same amount. The table uses one 10 mg vial and a 1 mg example amount (not a dose).
| Water added (mL) | mg per mL | Units for 1 mg | mL for 1 mg |
|---|---|---|---|
| 1 | 10 | 10 | 0.1 |
| 2 | 5 | 20 | 0.2 |
| 2.5 | 4 | 25 | 0.25 |
| 4 | 2.5 | 40 | 0.4 |
Each cell is computed by the same code as the calculators. Adding more water does not change how much compound is in the vial. It only spreads it over more mL.
Check before you draw
- Confirm the syringe says U-100. Other scales (U-50, U-40) give different numbers for the same mL. See units to mg.
- Keep the vial unit and the dose unit the same, or convert mg and mcg first. Never convert IU.
- Compare your result with the pharmacy label, or ask your pharmacist to check it with you.
- How long a mixed vial may be kept comes from the label. See how long does a mixed vial last.
Related calculators
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- hCG units calculator (IU)
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Related guides
- Convert mg to units on a U-100 syringe
- How long does a mixed vial last?
- Injection sites and rotation
- How to track peptide injections
- All guides
Vialist Log does this automatically
Enter the vial amount and the water you added once. Vialist works out the concentration, shows the units to draw for the dose your prescriber gave you, and counts the vial down as you log.