Calculating drug dosages is a very important skill for those in health care occupations. It's critical to know how to calculate the doses properly; improper doses cannot only not heal the patient, but could cause major complications and even kill them. The main concern in calculating dosages is to do it carefully, without rushing, and to make sure you convert measurement units properly. Converting units improperly is one of the main causes for administering medication inappropriately. However, there is an easy way to make the calculations once you're familiar with it.
Things You'll Need
 Pencil
 Paper
 Calculator
Organize Information, Use Formula, and Solve

Determine what dosage is desired and what form you have the medication in. For an easy example, say that the dose is supposed to be 500 mg of medication A. You have medication A in 100 mg tablets.

Use the formula: (desired amount)/(amount available) x (quantity), or written simply: D/A x Q = X (unknown amount)
So, for the example of 500 mg of medication available in 100 mg tablets, we'd set it up as follows:
500 mg/100 mg x 1 tablet = X
Solve the equation and you get 5 tablets.

Now for a slightly harder example, involving IVs:
Let's say that the doctor orders 45 mg of medication B to be administered. You have medication B available in IV form in the strength of 20 mg/5 mL (20 mg of medication per 5 mL of solution).Set the equation up as:
45 mg/20 mg x 5 mL = X
Solve the equation and you get 11.25 mL of solution needed to administer 45 mg of medication B.

Always consider whether your answer makes sense. If you put the numbers into the equation incorrectly, you can come up with radically different numbers, but they won't make sense if you think through a rough idea of what the answer should be. As an example, in the first example, if you reversed the 500 and 100, your answer would be to give 1/5 of a tablet, which does not make logical sense if you consider the problem.
Tips & Warnings
 Check and doublecheck yourself before actually administering the medication.
References
 Calculation of Drug Dosages; Sheila J. Ogden; 2007