Dosage calculation - Formula method: Nursing pharmacology

As the nurse, you’ll perform medication calculations to ensure the correct dose of a prescribed medication is administered. The three methods for drug calculations are basic formula, also called Desired over Have; ratio and proportion; and dimensional analysis.  Let’s take a deeper look into the basic formula method.  

As with the other methods for drug calculation, the three components involved in the basic formula method are D for the Desired dose, or dose ordered by the health care provider; H for Have, or the dosage you have available; and V, for Vehicle or the form in which the medication comes, like tablets or liquid.  

The basic formula method is set up like this:  X = Desired x Vehicle / Have

Now, before the drug dosage can be calculated, all units of measurement must be converted into one system, so they’re all the same. For example, if the medication is ordered in grams and comes in milligrams, then grams are converted to milligrams or milligrams are converted to grams. To do this, you'll use conversion factors, which are simply equivalents of measurements, like 1 gram equals 1000 mg or 1 liter equals 1000 mL.   Let’s look at some drug calculations using the basic formula method. 

First, you’ll read the order, which is: diphenhydramine 25 mg IV one time.  

Now, let’s look at the medication label.  

Since the medication comes in 50 mg per 1 mL, you’ll calculate how many mL you should administer to achieve the desired dose. To do this, first identify your components, Desired, Have, and Vehicle. 

In this case, D is 25 mg, which was obtained from the health care provider’s order. H is 50 mg, which was obtained from the medication label. And V, is 1 mL, which was also obtained from the medication label.  

Next, you’ll determine if a conversion factor is required. To determine this, compare the units of D with the units of H, and see if they’re the same. Because both D and H are in milligrams, no conversion factor is needed.  

Now, set up your equation, where X is the dose you’ll administer, written out like this: 

X volume in milliliters equals Desired over Have multiplied by Vehicle

Now, plug in the values to get X mL equals 25 mg multiplied by 1 mL divided by 50 mg. 

Remember, you can cross out the units of the numerator that match the units of the denominator, because they cancel out. 

Next, you’ll multiply 25 by 1 mL and divide by 50 to solve for X. 

The answer is 0.5 mL, meaning you'll administer 0.5 mL of diphenhydramine to achieve the ordered dose of 25 mg. 

Let’s look at another example, when a conversion factor is required. 

First, you’ll read the order, which is: cyanocobalamin 0.1 mg SUBQ once every three days.  

Now let’s look at the medication label.

Since the medication comes in 1000 mcg per 1 mL, you’ll calculate how many mL you should administer to achieve the desired dose. To do this, first identify your components, Desired, Have, and Vehicle. 

In this case, D is 0.1 mg, which was obtained from the health care provider’s order. H is 1000 mcg, which was obtained from the medication label. And V is 1 mL, which was also obtained from the label.  

Next, you’ll determine if a conversion factor is required. Because D is in milligrams and H is in micrograms, a conversion factor is needed.  In this case, you’ll use the following conversion factor for mcg to mg: 

1 mg equals 1000 mcg