Master Your Drug Calculations

  1. As I am sure you are all aware, it is extremely important that you do your dosage calculations in a safe and effective manner.

    The keys to success in this field are:

    • Knowing what is happening in the calculation rather than blindly following a formula.
    • Setting up your calculations mathematically correct.

    5 g (1000 mg/g) = 5000 mg is correct.
    5 g x 1000 = 5000 mg is incorrect.

    Unit conversions, dosage calculations, percent problems, and IV flow rate problems can all be solved using a simple and safe method called dimensional analysis (DA). If you take a couple of evenings to learn this method, you will save yourself hours of trying to learn a long list of formulas.

    These problems all have the same three parts:

    • The Units of the Answer: Think of it as the destination.
    • A Given: This is what is given to start the problem and what is changed into the answer.
    • One or More Ratios: These are the tools used to change the units of the given into the units of the answer.


    Examples:

    Unit Conversion: How many mL in 3.5 L.
    3.5 L is the given. mL is the unit of the answer. The ratio is 1000 mL/L
    3.5 L (1000 mL/L) = 3500 mL
    L cancel out and you are left with mL in the answer.

    Dosage Calculation: A patient is ordered 500 mg of a drug which is available in an oral suspension of 250 mg/5 mL. How many mL will you administer?
    500 mg is the given. mL is the unit of the answer. The ratio is 250 mg/5 mL
    500 mg (5 mL/250 mg) = 10 mL
    In this case, we had to flip the ratio upside down, which is permissible.

    Percent Problem: Convert 0.458 to a percent.
    0.458 is the given, % is the unit of the answer and 100% is the ratio.
    0.458 (100%) = 45.8%

    IV Flow Rate: An IV is running at 30 mL/h with a drop factor of 20 (20 drops/mL), how many drops/min is that?
    30 mL/h is the given. Drops/min are the units of the answer. 20 drops/mL and 60 min/h are the ratios.
    30 mL/h (1 h/60 min) (20 drops/mL) = 10 drops/min

    Unfortunately, there is not enough space here to go into all the details of this method, but you can PM me and I will send you some study material. Also, I am always glad to help with specific calculation questions.

    Brad Wojcik, PharmD
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