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

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