**How To Do Medication Dosage Calculations**

**Introduction**

** **Learning how to do medication dosage calculations is a fundamental skill for nurses, paramedics and other healthcare professions. The question of “how much do I give?” is a more complex question than just looking at the dosage required. There are many different sizes of pills and concentrations of liquids. It can be challenging to understand how to do these calculations however it is absolutely necessary. On most medication administration records, it will tell you how many “tabs” or “ml” to give however often times new medication orders are often hand written onto medication administration records without the number of “tabs” or “ml” so it is important to know how to do it yourself. Let’s get started.

**Definitions**

Before we begin let’s talk about a few very important terms or definitions that may seem simple however are important to correctly understand in order for this all to make sense.

**Route**: How the medication is entering the body. Is it by mouth (PO)? Is it intravenously (IV)? If is topically?

**Medication**: This is the actual compound that is entering the body that is completing the desired effect.

**Dose**: How much of this medication is being given to the patient.

**Vehicle**: The way the medication is carried into the body. Is it in a liquid? Is it in a tablet?

**The Formula**

**D – Desired Dose**: The desired dose is how much of the actual MEDICATION you want to give to the patient. Remember, medication is always measured as a unit of WEIGHT not volume.

**H- Amount On Hand**: This is how much the medication comes on the floor you are working. For example, the floor I work on only has metformin available in 500mg per tab concentration, therefore, the amount we have on hand is 500mg.

**V – Vehicle**: The vehicle can be difficult to truly understand but a good way of thinking about it is asking yourself, what does a vehicle do in real life? A vehicle is how you get from one point to another, whether it be a car, bike or scooter. In the medication world, a vehicle is how the medication gets into the body. Is it with tabled? Liquids? This will make more sense in the example questions.

Now that we have all three parts of the formula explained, lets do a practice question and compare the difference a change in vehicle makes.

**Practice question #1**: A patient is ordered 50mg of Dilantin PO BID. The unit only has the drug available in a 125mg/5ml concentration. How many millilitres do you give this patient per dose?

**Step One: Identifying The 3 Components of The Formula**

In order to be able to use a formula you will need to know how to fill it out. In order to do this, we need to know how to identify the amount required, amount on hand and the vehicle.

**Desired** **Dose**: The desired dose is what the patient is ordered. In this case, the patient is ordered 50mg of Dilantin, so the desired dose is 50mg.

**Amount on Hand**: The question states that the drub is available in a 125mg/5ml concentration. This is not the on hand however. The on hand refers to how much of the MEDICATION the unit has available per unit of vehicle. In this case, the on hand would ONLY be 125mg, NOT “125mg per 5ml”.

**Vehicle**: So how is this medication entering the individual? What does the 125 mg on hand dosage come in? The answer is 5ml. In this situation, you are able to get 125mg of Dilantin in a 5ml liquid. Therefore, the vehicle is 5ml. (Remember, the vehicle is never a measure of weight, it can be a measure of volume or tabs). A **VOLUME IS NOT A DOSE**.

Now that you have all 3 components of the formula, it should look like this:

**Step 2: Solve The Equation.**

To solve this just divide 50 by 124 (= 0.4) and multiply by 5 (=2ml). Therefore, the total amount given to this patient is 2ml.

Now let’s take a look at a very similar question.

**Practice Question #2**: A patient is ordered 50mg of Dilantin PO BID. The unit only has the drug available in a 125mg/10ml concentration. How many millilitres do you give this patient per dose?

A keen eye will notice that the only change from question number 1 to question number 2 is the vehicle. Regardless, lets work through this question to see what we get.

**Step One Identifying The 3 Components of The Formula**

** ****Desired** **Dose**: The desired dose is 50mg.

**Amount** **on** **Hand**: The amount on hand is 125mg.

**Vehicle**: The vehicle is 10ml.

Your formula should look like this:

**Step 2: Solve The Equation.**

50mg/125mg = 0.4. Multiply that by 10ml and you get 4ml. The patient was ordered the EXACT SAME dose as the previous question. Nothing was changed at all with how the physician ordered it. The only difference was the change in what the hospital had available. The change in vehicle can make a huge difference in how much you are going to give to a patient. Try these practice questions to test your knowledge.