Master Your Drug Calculations BEFORE You Get to Nursing School

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

    Master Your Drug Calculations BEFORE You Get to Nursing School

    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
    Last edit by Joe V on Jun 14
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    19 Comments

  3. by   bjwojcik
    Also, here is a free copy of my book which you may find helpful. It is copyrighted, but I am the owner of the copyright, so you are free to print it out.

    -Brad Wojcik, PharmD
    Attached Files
  4. by   Wiggly Litchi
    This is actually really handy Thanks!
  5. by   RainbowSprinkles
    Very helpful, thank you!
  6. by   Eden111
    can you please explain the last problem. the IV flow rate. TIA.
  7. by   ItsThatJenGirl
    Quote from ocean43
    can you please explain the last problem. the IV flow rate. TIA.
    If the drip is running 30mL/hour, then it's 0.5mL per minute. If the rate is 20 drops per mL, then you would 10 drops/min.

    30mL/60 mins = 0.5mL per minute, 20 drops/mL/0.5mL per minute = 10 drops per minute.

    Does that help at all?
  8. by   bjwojcik
    This is the way I like to set them up. You are starting with 30 mL/h and have to end up with gtt/min. Now you can see that you have to change mL to drops and h to min. The ratios of 20 gtt /mL and 60 min/h are the tools you use to make those changes. You have to line the ratios up so that the units you don't want in the answer get canceled out. 30 mL/h (20 gtt /mL) (1 h/60 min) =10 gtt/min. You can download the book for a more in depth explanation and practice problems. The way Jen explained it is good also. You should understand both ways. Just an FYI that the ratios always equal 1, so you are not really changing the value, just renaming it.
  9. by   Eden111
    thank you both very much. I start the program in few months and figured I take your advice on practicing dosage calculation til then is there any books you can recommend ?
  10. by   bjwojcik
    If you look under the comments you will see a pdf called dosage calculations. I wrote it for pharmacy technician students, but chapter 2 is pretty much all you need. It will teach you how to do the problems without all the formulas. I used to teach this subject and used this method and the students picked it up quickly.
  11. by   OneWhisper
    LOVE this post!! This will be so helpful to students!! I wish this was around when I was in nursing school and learning dosage calculations!!

    I DEFINITELY recommend dimensional analysis! So many students try doing the formula method, which involves needing to know formulas for each kind of calculation (mL/hr, gtts/min, etc.). It is soooo much easier using dimensional analysis because you can use it for EVERY kind of calculation that will be thrown at you. Once you learn to set it up, you will never have an issue. The other thing I love about dimensional analysis is that you follow a path - you go from what is ordered to what is available (your answer). You just follow along the path, using conversion factors as necessary, until you get to your answer!! It really helps to know that you did things correctly and helps you be confident in your answer!
  12. by   bjwojcik
    It is refreshing to see that there are others who understand that dimensional analysis is the easiest, safest way to do these calculations. I have taught this subject and written a book on it. I have also researched how other professions do these calculations. Chemists, physicists and engineers all use dimensional analysis, but for some reason the medical community either has a plethora of formulas they use or break the problems up into a bunch of ratio proportional problems. Sometimes it get frustrating trying to convince people there is a simple way to do these problems.
  13. by   lgarza825
    YIKES, this scares me! I just got accepted into the nursing program and this is def one of the things I am most worried about. Thank you for this post! I will for sure be getting this book and looking forward to more of your posts
  14. by   bjwojcik
    Let me know if you have any questions. I am retired and pretty much just ski, go to the gym and try to help students where I can. Once you realize that you are only changing the units by multiplying by various forms of 1 (the ratios) to get the units you want, these calculations will be a breeze.
    -Brad

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