Start With the Three Numbers
Every drug calculation reduces to three values: the dose ordered, the concentration on hand, and the volume to give. If you can identify which one is missing, you can solve the problem. Write the problem out on your scratch paper before touching the calculator.
The Basic Formula
Volume to administer equals the desired dose divided by the concentration on hand. For example, if the order is 0.3 mg of epinephrine and you have 1 mg/mL (1:1,000), you divide 0.3 by 1 and give 0.3 mL. For weight-based doses, multiply the dose per kilogram by the patient's weight in kilograms first, then apply the formula.
Avoid the Classic Mistakes
The most common error is forgetting to convert pounds to kilograms. Divide pounds by 2.2 before doing anything else. The second most common error is mixing units such as micrograms and milligrams. Convert everything to the same unit, then check whether your answer is reasonable. A 5 mL dose of epinephrine should immediately feel wrong.