How reconstitution math works

Updated October 2026 · Educational math explainer, not medical advice.

The numbers below are made-up examples to show the arithmetic. They are not dose suggestions. Always use the dose your prescriber gave you and the instructions from your pharmacist.

Made-up example: 5 mg powder + 2 mL liquid → 2.5 mg/mL; 0.25 mg ÷ 2.5 = 0.10 mL = 10 units. Not a dose or mixing instruction.

Some medications come as a dry powder in a vial. Before use, a measured amount of liquid (as your pharmacist instructs) is added. That step is called reconstitution. Your pharmacist or prescriber tells you how much liquid to add. The math afterwards only takes three steps.

Step 1: Concentration

Concentration is how much medication is in each mL of liquid.

concentration (mg/mL) = amount in vial (mg) ÷ liquid added (mL)

Example: 5 mg ÷ 2 mL = 2.5 mg/mL

Step 2: Volume to draw

volume (mL) = prescribed dose (mg) ÷ concentration (mg/mL)

Example: 0.25 mg ÷ 2.5 mg/mL = 0.10 mL

If your dose is written in mcg (micrograms), divide by 1,000 first: 250 mcg = 0.25 mg. Mixing up mg and mcg is a common and serious error, and it's a 1,000× mistake. VialMath flags some numbers that look like the wrong unit.

Step 3: Units on a U-100 syringe

units = volume (mL) × 100

Example: 0.10 mL × 100 = 10 units

On a U-100 syringe, 100 units equals 1 mL. See reading a U-100 syringe.

How many doses are in the vial?

doses ≈ amount in vial (mg) ÷ dose (mg), rounded down

Example: 5 mg ÷ 0.25 mg = about 20

Real-world yield is often a bit lower, because some liquid stays in the syringe and needle (dead space).

Checks worth doing every time

Is the dose smaller than the whole vial? If not, a unit is probably wrong.
Does the draw fit your syringe? 68 units won't fit a 50-unit syringe.
Is the draw big enough to measure? Under about 2 units is hard to measure accurately.
Does the result match what your pharmacist told you? If not, ask before you use it.

Try it with your own numbers: open the calculator. It shows every step so you can check the math.

This tool does arithmetic on numbers you enter. It does not recommend doses. Confirm with your prescriber or pharmacist.