Or you can do the math....
Dose (mg) = Volume (mL) x Concentration (mg/mL)
If I reconstitute a 20mg vial with 4 mL of water, that gives me 5mg/mL. Say I wanted a 1mg dose, we get the following:
1mg = volume (mL) x 5 mg/mL
We will then solve for volume by dividing both sides of the equation by 5. That gives us 0.2 mL for volume. Multiply that by 100 to get 20 units.

