Dosage Calculation 101

Do you need to know how to do math in order to be a nurse? The short answer is YES!

Did you have to ace trigonometry and algebra in high school to be a good nurse? No. In fact, I HATED math in high school and early my early college days. The fact that I was so bad at math was concerning to me when I found out I would have to learn complex dosage calculations in order to pass nursing school. BUT, let me tell you a secret…learning “math” for nursing school is as simple as knowing how to set up your problem. When you learn these FIVE simple steps I am going to discuss in this article, you will NEVER miss another nursing math problem. This method is called dimensional analysis and it makes dosage calculations SO EASY! Let’s get to it!

1. To work nursing math, you must know your metric conversions.

When I say metric conversions, I am referring to knowing how to convert grams to milligrams and milligrams to micrograms, etc.

Use this table for a quick reference:

· 1 Kg = 1000 Gm

· 1 Gm = 1000 mg

· 1 mg = 1000 mcg

· 1 tsp = 5 mL

· 1 Tbsp = 15 mL

· 1 L = 1000 mL

· 3 tsp = 1 Tbsp

· 30 mL = 1 oz

· 2.2 lbs = 1 Kg

2. To work nursing math, you must look for what you are GIVEN and what you WANT TO CONVERT TO.

For example, 5500 Gm = Kg?- Here, we want to know how many Kg are in 5500 Gm. We are CONVERTING to Kg. What we are GIVEN is the amount of Gm, which equals 5500.

3. To work nursing math, you must put WHAT YOU ARE GIVEN on top.

GIVEN = 5500 Gm. What you are given always goes on top of a fraction. Our example will look something like this: 5500 Gm/ 1. Since Gm is what we are GIVEN, it will go on top over a 1. We cannot put the given amount over any other conversion yet because we haven’t actually DONE any converting. We have to use 1 in this example in order for our GIVEN to go on the TOP.

4. To work nursing math, you must put WHAT YOU ARE CONVERTING TO on top.

We want to CONVERT to Kg. Kg goes on top. We know that 1 Kg = 1000 Gm.

The next part of our problem will look like this: 5500 Gm/ 1 x 1 Kg/ 1000 Gm. See how what we are GIVEN and what we are CONVERTING to are both on top? Always remember this. We are trying to get rid of what we are GIVEN and end up with what we are CONVERTING to.

5. To work nursing math, you must make sure that matching units cancel each other out.

In this case we want to get rid of Gm. So, we will multiply the two fractions, canceling out matching units. 5500 Gm/ 1 x 1 Kg/ 1000 Gm. We have Gm on top in the first fraction and Gm on the bottom in the second fraction so the Gm cancel each other out. We are then left with 5500 / 1000 which equals 5.5 Kg.

Let’s apply this method to some more complicated calculations!

Order: Demerol 150 mg IM

Available: Demerol 100mg/2ml.

The nurse would give Demerol __________mL.

We will skip to Step 2 here which is to identify what you are GIVEN and what you want to CONVERT to-GIVEN: 150 mg, CONVERT to: mL

Step 3 and 4: Put GIVEN and CONVERTING to on TOP- x

Step 5: Multiply the fractions, canceling out matching units. Make sure you will be left with what you are CONVERTING TO. = 2 mL

Let’s try another one…

Order: Dopamine 10 mcg/kg/min. The patient weighs 210 pounds.

Available: Dopamine 800mg/500mL.

The nurse will set the IV pump to infuse _________ mL/hr.

Step 2: GIVEN- Dopamine 10 mcg/kg/min, CONVERT to: mL/hr

HINT: With weight-based problems go ahead and calculate how many mcg per Kg the patient will receive by multiplying 10 x the patient’s weight in Kg. Since the patient weighs 210 lbs we divide by 2.2 to get 95.5 Kg (I rounded to the tenths but be sure to follow your program’s rounding rules). We will then multiply 95.5 Kg x 10 to get 955 mcg, so the patient is GIVEN 955 mcg/ min. NOW, we need to CONVERT this to mL/ hr.

Step 3 and 4: Put GIVEN and CONVERTING to on top- x

Ok so here we have 955 mcg on top because it is what is GIVEN. We also have 500 mL on top because we are CONVERTING to mL/ hr. BUT, we have some extra steps because mcg and mg do not cancel out. Also, remember that we are looking for mL per HOUR; not mL per minute, so we have to get rid of minutes also. Enter our metric conversions. Now remember, we MUST end up with what we are converting to, so mL/ hr must never cancel out. So let’s set up our problem.

Step 5: Make sure that you will be left with what you are CONVERTING TO and cancel out matching units.

mL/hr= x x = 28, 650,000/ 800,000= 35.8 mL/ hr (again, make sure to use your program’s rounding rules). See how we did that? It’s just a matter of setting up your problem so that EVERYTHING except what you are CONVERTING to cancels each other out!

How about one more?

Order: Lidocaine 0.5 mg/min.

Available: Lidocaine 2gm/500mL.

The nurse will set the IV pump to infuse: _____________________mL/hr

Step 2: GIVEN- 0.5mg/ min, CONVERTING TO – mL/ hr

Step 3 and 4: Put GIVEN and CONVERTING TO on top:

mL/ hr= x x x

Step 5: Make sure that you will be left with what you are CONVERTING TO and cancel out similar units.

15000/ 2000= 7.5 mL/hr

Now go forth and pass your med calc exam!!