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7 Must-Know Tips for Dosage Calculation

We all cringe when we see medication calculations on the NCLEX or even in real life. Not only are nurses asked to do complex dose calculations - we often are doing them in real life under extreme time pressure or professional stress. These are not optimum conditions for complex math. Here are some hints to keep your calculations straight.

1. Write it out.
Sounds simple, but it can really help. When you are doing a dose calculation, write it down on a piece of paper. It can really help to see errors and work it out.

2. Memorize common conversions.
You will find that there are common conversions that you will use ALL the time. Temperature from Celsius (C) to Fahrenheit (F) and back. Pounds (lbs) to kilograms (kg) and back. Milliliters (ml) to ounces (oz) and back. Commit these to memory. And install an app on your phone to use in real life (you cannot use it on the NCLEX).

C = 5/9 (F-32)
F = 9/5 (C+32)

lbs / 2.2 = kilograms
kg x 2.2 = pounds

1 fl oz. = 29.57 ml
1ml = 0.033814 fl oz.

3. Understand the metric system
If you don't feel that you are strong in understanding that there is 1000 milligrams in 1 gram (0.001 mg=1g) and 1000 grams in 1 kilogram (1000g = 1kg) and how to convert back and forth in your sleep. Then pay a nurse tutor for an hour and get a rock-solid understanding of this system. It can make or break you in nursing. And making an error can kill a patient and your career.

4. Review the list of abbreviations to AVOID
I can bet you 99% that there will be at least one NCLEX question that touches on this list. Review what you should and should not see or use on an order. A comprehensive list can be found at the Institute for Safe Medicine (https://www.ismp.org/recommendations/error-prone-abbreviations-list)

5. Everything I learned about dose calculation I learned in high school chemistry...
So I had this awesome high school chemistry teacher named Mr. Campbell. He pretty much gave me a great nursing foundation for dosage calculation because he taught me this method for calculation called "the factor-label method of dosage calculation". Here's how you do it.

Say the nurse has an order to give a patient 300mg of Zantac. Zantac is a 15mg/ml solution. How many mls' should the nurse give?

  1. Ask yourself "What do I want to know at the end?" = how many ml?
  2. Ask yourself "What am I starting with?" = 300mg
  3. Write it out: 300mg x (1 ml / 15mg) = 20 ml
  4. Cross out the units that cancel - in this example, you can cross mg out on the top and bottom and all you are left with are ml.
  5. Ask yourself "Does the result make sense?" = ...I started with a large amount of mg, the concentration of the drug has a lot of mg in 1 ml so the answer I get should be smaller than what I started with - and it is. This common-sense double-check of "does it make sense" can really help your answers to be accurate every time.

There are lots of great examples on the internet of how to do factor-label conversion well. It can really help nurses make sure that they are getting the correct units. Search around if you need more examples.

Practice with this one (the right answer is at the end of the blog):
Order: Administer 20 mg per kg of amoxicillin every 12 hours for a child who weighs 15 kg. If the amoxicillin is 250mg/5ml then how many mls' should be given per dose?

6. Use a calculator.
Even if you are a math genius. When you are under pressure and trying to go quickly, that is when you can make mistakes. Use a calculator to at least double-check your math. Remember that testing sites are not guaranteed to provide a calculator, so don't forget to bring yours to the exam unless otherwise specified by the testing site.

7. Double check.
On the test - you have to double-check yourself. Go through it again. In real life, ask that someone double-check with you. We are actually required to double verify MANY high-risk medications and pediatric dosages.

You would multiply 15kg x (20mg/kg) = 300 mg x (5ml/250mg) = 6 mL