Reconstitution calculator

How the numbers work

Four steps, worked through on a 5 mg vial with 2 mL of water and a 250 mcg draw.

1. 1

   Concentration is the vial content divided by the water added

   2 mL of bacteriostatic water into a 5 mg vial gives 2.5 mg per mL. The water volume is the only part of this you choose, and it cannot be changed once the powder is wet.

2. 2

   Volume per draw is the amount you want divided by that concentration

   A 250 mcg draw is 0.25 mg, and 0.25 divided by 2.5 is 0.1 mL.

3. 3

   Units are millilitres times 100

   A U-100 syringe reads 100 units per millilitre, so one unit is 0.01 mL and 0.1 mL is 10 units. The zero graduation sits at the needle end.

4. 4

   Draws per vial is whole draws only

   5 mg of content divided by a 0.25 mg draw is 20 draws. Divide and round down — a part-draw left at the bottom is not a draw.

Common questions

How much bacteriostatic water goes into a 5 mg vial?

Any volume works, because it sets the concentration rather than the amount. 2 mL into a 5 mg vial gives 2.5 mg per mL, so a 250 mcg draw is 10 units on a U-100 syringe. 1 mL gives 5 mg per mL and the same draw is 5 units. Pick the volume that puts your usual draw on a number you can read.

How do I convert milligrams to syringe units?

Divide the amount per draw by the concentration to get millilitres, then multiply by 100. A U-100 syringe reads 100 units per millilitre, so one unit is 0.01 mL.

What is the concentration after reconstitution?

The vial content divided by the volume of bacteriostatic water added. A 10 mg vial with 2 mL of water is 5 mg per mL.

How many draws does one vial give?

The vial content divided by the amount per draw, rounded down to a whole number. A 5 mg vial drawn at 250 mcg gives 20 draws; the remainder stays in the vial.

Why does a pen show a different number than I asked for?

A pen dials in whole clicks, so a volume that falls between two clicks cannot be selected. The figure shown is the click the pen can actually reach, not the one requested.

The units come out very small. What should I change?

Too few units to read means the concentration is too high for that draw. Adding more water next time spreads the same amount over more volume, which moves the draw onto a part of the barrel that can be measured.