# Master Your Drug Calculations BEFORE You Get to Nursing School

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. Nursing Students General Students Knowledge

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## Keys to Success:

• 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.

Check Out These 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.

I love dimensional analysis. It came in quite handy when I took Chemistry because I already understood how to use it. It’s one of the simplest forms of math, imo. This is a great resource!

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As someone who loves math and the sciences, the way we were taught (but I didn't do) to do dosage calculations in nursing school made me cringe! Dimensional analysis is so much easier because it makes sense! You can't make a mistake with your units when you understand how to arrange them.

Thanks for shining a light on this!

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Specializes in Student.

Thank you so much!  This will be a huge help!

R

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